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Donor–acceptor template systems in vitro were designed to test mechanisms of minus strand transfer of human immunodeficiency virus 1 (HIV-1). Donor RNA D199, extending from the 5' end of the HIV-1 genome to the primer binding site (PBS), promoted transfer to only 35% with an acceptor RNA representing the 3' terminal 97 nucleotides. Whereas donor RNA D520, including an additional 321 nucleotides 3' of PBS, exhibited 75% transfer. Both donors transferred through an invasion driven pathway, but transfer was stimulated by the folding structure resulting from the extra segment in D520. In this study, the significance of interaction between the tRNAlys3 primer and U3 was examined. Measurements utilizing acceptors having or lacking the U3 region complementary with tRNAlys3 indicated that a tRNAlys3-U3 interaction compensated for inefficient acceptor invasion observed with D199. Stimulation presumably occurred because binding to tRNAlys3 increased the proximity of the acceptor to elongated cDNA, improving transfer to 78% efficiency with D199, and even higher to 85% with D520. The stimulation did not require natural viral sequences, but could be achieved by substituting the original U3 sequence with an equal length sequence that binds a different region of the tRNAlys3. Comparison between acceptors sharing the natural region for tRNAlys3-U3 interaction but having or lacking the acceptor invasion site demonstrated that tRNAlys3-U3 interaction and acceptor invasion cooperate for maximal stimulation. Overall observations suggest that both proximity and invasion mechanisms are applied successively by HIV-1 for efficient minus strand transfer.
During retroviral replication, single stranded viral genomic RNA is converted into a double stranded viral DNA by the virus-encoded enzyme reverse transcriptase (RT), in the pathway termed reverse transcription (1, 2). Reverse transcription of retroviruses, including HIV-1, is initiated by a virion-packaged cellular tRNA primer annealed to the PBS located near the 5' end of genomic RNA. RT-catalyzed DNA synthesis proceeds toward the 5' end of the viral genome generating the 199 nucleotide-long minus strand strong stop DNA (−sssDNA). Meanwhile, the RNase H activity of the RT makes endonucleolytic cleavages on the copied RNA genome, facilitating the −sssDNA to translocate from the 5' repeat (R) region of the viral genome to the 3' R. This step is termed −sssDNA transfer. Minus strand DNA synthesis then resumes through a series of steps to produce the full-length double stranded DNA (3, 4).
Extensive studies have demonstrated that nucleocapsid protein (NC) significantly stimulates minus strand transfer in vitro (5). NC is a nucleic acid chaperone (6, 7). It was suggested that NC enhances the rate of annealing between complementary nucleic acid strands, more specifically, the annealing between −sssDNA and 3' end of the viral genome through R region complementarity during minus strand synthesis and transfer (8–12). Also, it was reported that NC inhibits DNA self-priming of the −sssDNA, a pathway leading to a dead-end product, in a manner that greatly improves transfer (10, 11, 13–17).
Reconstituted systems in vitro, with donor RNA templates representing the 5' end of the viral genome and acceptor RNA templates representing the 3' end of the viral genome, were designed and used to probe possible mechanisms of HIV-1 minus strand transfer (18–24). When the homology between donor and acceptor RNAs consisted of only a short segment of the 5'-most sequence on the donor, RT RNase H cleavages at the very end of donor facilitated the −sssDNA to dissociate from the donor and transfer to the acceptor (25–28), a mechanism termed the terminus transfer pathway (19, 22). When the homology between donor and acceptor was longer, RNase H cleavages occurring early on in the primer extension reaction generated gaps in the donor (29), promoting the acceptor to invade and hybridize to the cDNA (30). A region at the base of the trans-activation-responsive (TAR) hairpin was particularly effective at pausing RT synthesis, allowing the RNase H to clear an area thought to be the primary invasion site. The acceptor-cDNA hybrid then propagated, finally reaching the cDNA 3’ terminus, which transferred to complete the reaction. This mechanism has been termed the acceptor invasion driven pathway (20, 21, 23). Additionally, we recently designed experiments showing that as a result of successful acceptor invasion, the increased local concentration of complementary acceptor sequences at cDNA 3' terminus facilitated direct transfer through a proximity mechanism (24).
In retroviruses, a host encoded tRNA is utilized as the primer by RT to initiate −sssDNA synthesis (4). Some retroviruses can readily be mutated to use alternative type of tRNA. It was observed that murine leukemia viruses (MLV) harboring specific mutations in the PBS matching the 3' end of alternative tRNA primers replicated with no great difference in efficiency in vivo. This suggested that the PBS-tRNA primer complementarity rather than features of the unique natural tRNA primer is essential for facile replication of MLV (31, 32). With HIV-1, although various primers could be utilized in reconstituted systems in vitro (33), the natural tRNAlys3 conferred a substantial advantage in vivo. In contrast to what was observed with MLV, different PBS-tRNA primer combinations in HIV-1 led to significant lags in viral replication. These observations suggest that the natural tRNAlys3 makes unique contacts that are essential for optimal viral growth of HIV-1 (34–37).
Several studies have demonstrated that interactions between tRNA and the viral genome outside of PBS are important for retroviral replication (33, 38–46). For instance, it was suggested that for avian sarcoma and leucosis virus, an interaction between the TψC loop of the tRNAtrp and the U5 region of viral genomic RNA is essential for efficient initiation of reverse transcription (38). For HIV-1, tRNAlys3 is capable of interacting with viral genomic RNA at multiple sites (47, 48). The interaction between the anticodon loop of tRNAlys3 and the A-rich loop upstream of PBS in the viral genome is essential for replication of HIV-1 (49), and additional interactions between tRNAlys3 and the viral genome are supported by structure probing analysis of the tRNAlys3– genomic RNA binary complex (48). In addition, it was observed that the anticodon loop and the D-loop of tRNAlys3 interact with HIV-1 RT (50–52), and such tRNAlys3-RT interaction stimulates both the DNA polymerase and RNase H activity of RT (53). Moreover, the unwinding of the highly structured tRNAlys3 primer is promoted by NC. It was suggested that NC facilitates the interaction of the tRNAlys3 primer with the PBS to initiate reverse transcription (54). The overall results indicate that interactions of tRNAlys3 with RT, NC, and the viral genome outside of PBS are essential for replication of HIV-1 (47–55).
Previously in a DNA-primed transfer reaction in vitro, we showed that donor RNA D199, which included the minimal region required for minus strand synthesis, promoted transfer to only 35% with an acceptor RNA named A97h. This acceptor had homology to the 5'-most 97 nucleotides (nts) of the donor. The sequence includes the putative invasion site at the base of the TAR hairpin, and therefore allows invasion driven transfer. Donor RNA D520, which included an additional 321 nts 3' of PBS, transferred at an efficiency of 75%. We determined that both donors promoted transfer primarily through the invasion driven pathway (23). For D520, however, the folding configuration of the longer donor promoted much more effective invasion and terminus transfer. We also found evidence that efficient acceptor invasion raises the local concentration of acceptor for effective transfer through a second proximity effect pathway in addition to the originally conceived branch migration pathway of propagation (24).
Marquet and co-workers proposed that interaction of the tRNAlys3 primer with U3 greatly enhanced minus strand transfer of HIV-1 in vitro. It was observed that a nonanucleotide segment, positioned at 9091–9099 in the U3 region of the Mal strain of HIV-1, is exactly complementary to nucleotides 38–46 of tRNAlys3 (Fig 1). In a tRNAlys3 primed transfer reaction, there was a significant decrease in transfer when using acceptors in which that nonanucleotide segment was mutated or deleted. This implies that the tRNAlys3-U3 interaction contributes to more efficient transfer during viral replication (56). Since the nonanucleotide segment in U3 identified by the Marquet group is well conserved throughout different strains of HIV-1 (56), we investigated whether such tRNAlys3-U3 interaction (56) could also influence transfer in our D520 and D199 systems. The approach was to modify our systems to prime with tRNAlys3 and compare transfer efficiency when the acceptors had or lacked the nonanucleotide segment. We were particularly interested in determining how the tRNAlys3-U3 interaction influences invasion driven transfer, and whether the extra sequences in D520 would synergize with the U3 sequences to maximize transfer efficiency.
Recombinant HIV-1 reverse transcriptase (p66/p51 heterodimer) was purified as described previously (57, 58). HIV-1 NC (71 aa) was prepared as described previously (59, 60). DNA oligonucleotides were purchased from Integrated DNA Technologies, Inc (IDT) (Coralville, IA). The pNL4-3 molecular clone was obtained through the AIDS Research and Reference Reagent Program (Division of AIDS, National Institute of Allergy and Infectious Diseases, National Institute of Health). T7-MEGAshortscript high yield transcription kit was purchased from Ambion, Inc. (Austin, TX). The Platinum Taq DNA polymerase was purchased from Invitrogen (Carlsbad, CA). The purified tRNAlys3 from human placenta was purchased from Bio S&T (Lachine, Quebec, Canada). The 32P isotope was purchased from Perkin-Elmer Life Science (Boston, MA). All other enzymes, as well as the dNTP mixture were purchased from Roche Applied Science (Indianapolis, IN).
Genomic sequences of HIV-1 pNL4-3 strain were amplified by PCR using Taq DNA polymerase to generate DNA templates for the preparation of RNA samples. The donor RNA templates D199, D520 and acceptor RNA template A97h, A19h were generated by run-off transcription in vitro as described previously (23, 24). The acceptor RNA substrates A97h(-54), Δ-A97h(-54), mut-A97h(-54), A19h(-54), and mat-A97h(-54) were transcribed in vitro from a synthetic double-stranded DNA fragment using the Ambion T7-MEGAshortscript kit. The sequence of the forward PCR primer for A97h (-54) was 5'- TAA TAC GAC TCA CTA TAG GGA GTG GCG AGC CCT CAG ATG - 3', whereas the sequence of the reverse primers for A97h(-54) was 5'- TGA AGC ACT CAA GGC AAG CTT TAT TGA GGC - 3'. The sequence of the forward PCR primer for Δ-A97h(-54) was 5'- TAA TAC GAC TCA CTA TAG GGA GTG GCG AGG CTG CAT ATA AGC AGC TGC TTT TTG CCT GTA- 3', whereas the sequence of the reverse primers for Δ-A97h(-54) was 5'- TGA AGC ACT CAA GGC AAG CTT TAT TGA GGC - 3'. The sequence of the forward PCR primer for mut-A97h(-54) was 5'- TAA TAC GAC TCA CTA TAG GGA GTG GCG AGT ATA TAT ATG CTG CAT ATA AGC AGC TGC TTT TTG CC- 3', whereas the sequence of the reverse primers for mut-A97h(-54) was 5'- TGA AGC ACT CAA GGC AAG CTT TAT TGA GGC - 3'. The sequence of the forward PCR primer for A19h (-54) was 5'- TAA TAC GAC TCA CTA TAG GGA GTG GCG AGC CCT CAG ATG - 3', whereas the sequence of the reverse primers for A19h(-54) was 5'- GGT CTA ACC AGA GAG ACC CAG TAC AGG CA - 3'. The sequence of the forward primer for mat-A97h(-54) was 5'- TAA TAC GAC TCA CTA TAG GGA GTG GCG AGA GTC TGA TGG CTG CAT ATA AGC AGC TGC TTT TTG CC - 3', whereas the sequence of the reverse primer for mat-A97h(-54) was 5'- TGA AGC ACT CAA GGC AAG CTT TAT TGA GGC - 3'. In the NL4-3 strain of HIV-1, the nonanucleotide segment in U3 discovered by Marquet’s group occupies positions 9033–9040. All RNA templates used in the study were purified by denaturing PAGE and resuspended in 10 mM Tris-HCl (pH 8.0), 1 mM EDTA buffer. RNAs were quantified by using the Ribogreen assay (Molecular Probes, Eugene, OR).
Preparation of 5' radiolabeled tRNAlys3 was performed as described previously with slight modifications (61). A three fold excess of a 38 nt DNA oligonucleotide template (UP5), having the sequence 5'- TG TGG AAA ATC TCT AGC AGT GGC GCC CGA ACA GGG AC- 3', was heat-annealed to tRNAlys3 at 65°C for 5 min in buffer consisting of 50mM Tris-HCl (pH8.0) and 75mM KCl, and gradually cooled to 37°C. The reaction mixture was then treated with calf intestinal phosphatase (Roche) at 50°C for 60 min, extracted with phenol-chloroform, precipitated with ethanol, and resuspended in 50mM Tris-HCl (pH8.0) and 75mM KCl. Following incubation at 65 °C for another 5 min and slowly cooling to 37 °C, the reaction mixture was treated with γ-32P-ATP (6000 (222TBq) Ci/mmol). The radiolabeled tRNAlys3 and DNA template UP5 were separated by electrophoresis in a 6% sequencing gel. The tRNAlys3 template was eluted from the gel overnight, precipitated with ethanol, and resuspended in 10 mM Tris-HCl (pH 8.0), 1 mM EDTA buffer.
The sequence of DNA primer P1 is 5'- GCC CGG ATA GCT CAG ACG GAA GAG CAT CAG ACT CTT AAT CTG AGG GAC CAG GGT TCA AGT CCC TGT TCG GGC GCC A- 3'. The sequence of DNA primer P2 is 5'- GTC CCT GTT CGG GCG CCA - 3'. Preparation of 5' radiolabeled DNA primers P1 and P2 was performed as described previously (23, 24).
Reactions were performed as described previously with slight modifications (23, 24). The tRNAlys3 primer or DNA primers were heat annealed to donor RNA by incubating at 65°C for 5 min and slowly cooled to room temperature. Acceptor templates were then added and incubated with 150% NC (100% NC = 7 nt/NC) at 37°C for 5 min. Primer, donor and acceptor were mixed at a ratio of 1.5:1:3. RT was pre-bound to substrate at 37 °C for 5 min before reactions were initiated with MgCl2 and dNTPs. Final reactions contained 50 mM Tris-HCl (pH 8.0), 50 mM KCl, 1 mM DTT, 1 mM EDTA, 32 nM HIV-1 RT, 6 mM MgCl2 and 50 µM dNTPs. Reactions were incubated at 37 °C, and were terminated at appropriate time points with 2× termination dye (10 mM EDTA pH8.0, 90% formamide (v/v), and 0.1% each of xylene cyanole and bromophenol blue). Products were resolved by denaturing PAGE, and analyzed using a PhosphorImager (GE Healthcare) and ImageQuant software (version 2.1). Sizes of DNA products were estimated by using a 10 bp DNA ladder.
To analyze how the interaction between tRNAlys3 and U3 affected the minus strand transfer of D520 and D199, RNA acceptor template A97h (-54) was designed and a 5' radiolabeled tRNAlys3 primer was prepared (Fig. 1A). A97h(-54) was identical to acceptor template A97h, used in previous work (23, 24), with an additional 34 nt extension into U3 to include the special nonanucleotide segment complementary to nts 38–46 of tRNAlys3. Consequently, transfer products generated through acceptor A97h(-54) would be 34 nt longer than transfer products generated through acceptor A97h. For both of the transfer systems of D520 and D199, transfer products and donor extension products accumulated as the reaction proceeded when using acceptor template A97h (Fig. 1B). Surprisingly, when acceptor template A97h(-54) was used, donor extension products accumulated until about 16 min, and then faded out as they were chased into transfer products during the transfer reaction of D199. Moreover, donor extension products were barely visible even in early time points with D520. Transfer efficiencies of D520 or D199 with each acceptor template are presented in Fig. 1C in a graph of time dependence. All of the transfer reactions reached a plateau at 32 min. After the reactions were completed at that time, for donor D520, transfer efficiency increased from 40% with A97h to 85% with A97h(-54); for donor D199, transfer efficiency increased remarkably from 18% with A97h to 78% with A97h(-54). Clearly, the additional 34-nucleotide extension in the U3 region of acceptor A97h (-54) promoted transfer in both of the donor systems. This result is in thorough agreement with discoveries described by the Marquet group (56). It shows that the interaction between tRNAlys3 and U3 promotes a transfer efficiency two times greater than that using A97h for D520, and the transfer improvement goes up to four times for D199.
Another way to analyze the importance of tRNAlys3-U3 interaction was to delete or mutate the nonanucleotide segment in U3 responsible for interaction with tRNAlys3 and determine the effects on transfer. RNA acceptor templates Δ-A97h (-54) and mut-A97h(-54) were designed (Fig. 2A) for this purpose. Δ-A97h(-54) was generated by deleting the nonanucleotide segment from A97h(-54) and mut-A97h(-54) was generated by replacing the wild type nonanucleotide sequence in A97h(-54) with base substitutions.
Fig. 2B shows the transfer profile of D520 and D199 with each acceptor in a graph of time dependence. Clearly, the absence of the wild type nonanucleotide segment in U3 led to a significant decrease in synthesis of transfer products for both donors. When we summarized the final efficiencies after each transfer reaction was completed (Fig. 2D), we observed that for D520, transfer efficiency decreased from 85% with A97h(-54) to 46% with Δ-A97h(-54) and to 29% with mut-A97h(-54). The decrease was even greater with D199. Transfer efficiency dropped from 78% with A97h(-54) to 28% with Δ-A97h(-54) and to 12% with mut-A97h(-54). This result further supports the conclusion that the presence of tRNAlys3-U3 interaction is essential for efficient transfer of D520 or D199 with A97h(-54).
To examine whether the interaction of tRNAlys3 with U3 alone is sufficient to promote the highest transfer efficiency, or it needs to work with acceptor invasion involved with A97h(-54) for maximal stimulation, two acceptor templates, A19h and A19h(-54), were designed (Fig. 2A). A19h(-54) shared the same homologous region as A19h, but included an extended U3 region which incorporated the nonanucleotide segment complementary to nts 38–46 of tRNAlys3. Increases in transfer with A19h(-54), as opposed to A19h, are likely imposed by the presence of elongated U3 in A19h(-54). Also, A19h(-54) shared the same U3 region as A97h(-54), but was homologous to the first 19 nt of the 5' end of the donor templates, which could only allow for terminal transfer. Should any differences be observed in transfer ability between A19h(-54) and A97h(-54), they could only be attributed to an acceptor invasion driven pathway involved with A97h(-54).
Fig. 2C shows the transfer profile for each acceptor template with D199 or D520 in a graph showing time dependence, and the final transfer efficiencies after reactions were completed are summarized in Fig. 2D. For D520, transfer efficiency increased from 15% with A19h to 56% with A19h (-54), significantly less than the 85% observed with A97h(-54). For donor D199, there was hardly any transfer product throughout the reaction with A19h. Transfer efficiency increased from 2% with A19h to only 20% with A19h(-54). Evidently, compared to acceptor A19h, the presence of the extended U3 region in acceptor A19h(-54) promoted transfer for both of the donor systems through additional interaction with the tRNAlys3 primer. More importantly, for both donors, the significant difference in transfer between using A19h(-54) and using A97h(-54) suggests that the interaction of tRNAlys3 with U3 is not sufficient to promote the most efficient transfer. Overall observations show that the invasion driven interaction facilitated by A97h(-54) needs to cooperate with the interaction between tRNAlys3 and U3 to achieve the maximal efficiency.
To examine whether alternative stabilizing interactions between tRNAlys3 and U3 can also facilitate more efficient transfer as the tRNAlys3-U3 interaction suggested by Marquet group (56), acceptor mat-A97h(-54) was designed (Fig. 3A). The mat-A97h(-54) acceptor shared the same sequence as A97h(-54) except that the original nonanucleotide segment in U3 (5'-CCCUCAGAU-3'), complementary to nts 38–46 of tRNAlys3, was substituted with a segment (5'-AGUCUGAUG-3') complementary to nts 25–33 of tRNAlys3. We anticipated that any improvement in transfer efficiency with mat-A97h(-54) would likely be imposed by the tRNAlys3-U3 interaction conferred by the alternative sequence complementarity.
Fig. 3B shows the transfer profile of D520 and D199 with acceptor mat-A97h(-54) in a graph of time dependence. The final transfer efficiencies are summarized in Fig. 3C. After the transfer reactions were completed, D520 promoted an efficiency of about 85% and D199 promoted an efficiency of about 75%, almost to the same level as we observed previously when using acceptor A97h(-54). Clearly, the alternative sequence complementarity between mat-A97h(-54) and tRNAlys3 contributed to more efficient transfer. Presumably, the interaction of mat-A97h(-54) with tRNAlys3 brought the acceptor 5' terminus closer to the elongated cDNA and facilitated more effective transfer through an acceptor proximity effect, a similar mechanism as was observed with A97h(-54). These results suggest that the interaction of tRNAlys3-U3 rather than the specific wild type sequence of the substrates is the essential determinant for efficient minus strand transfer.
It was suggested that tRNAlys3 folds into a secondary structure and interacts with HIV-1 genomic RNA at multiple positions (48, 55, 62). To examine whether the folding of tRNAlys3 is important for the interaction with U3 that facilitates minus strand transfer, a DNA primer P1, which was of exactly the same sequence as tRNAlys3, was prepared. Presumably, DNA primer P1 could interact with acceptors A97h(-54) and mat-A97h(-54) via sequence complementarity. Moreover, any differences in the folding of P1 compared to tRNAlys3 would lead to primer-acceptor interactions of different stability, ultimately resulting in different transfer efficiency. Also, as a control, a second DNA primer P2, which represented the first 18 nt of the 3' terminus of tRNAlys3 and was only complementary to PBS on donor, was designed. Compared to primer P1, P2 was not capable of interacting with the U3 sequences in the acceptor RNA. Three acceptor templates A97h, A97h(-54), and mat-A97h(-54) were utilized in this study (Fig. 3A).
Transfer profiles of each transfer reaction are presented in a graph showing time dependence in Fig. 3D, and the final efficiencies of each transfer reaction are summarized in Fig. 3E. When using the DNA primer P1 with D520, no differences between results with different acceptors were observed. Since the presence of acceptor A97h already promoted a transfer efficiency of about 78%, any possible any small increases in transfer with either A97h(-54) or mat-A97h(-54) resulted from P1–U3 interaction might be hard to detect. For D199, however, we observed a transfer efficiency of about 42% with A97h, 44% with A97h(-54), and 43% with mat-A97h(-54). Clearly, the presence of P1–U3 interaction conferred by A97h(-54) and mat-A97h(-54) had no significant effect on transfer reaction of D199. Also, for all of the three acceptors used in this experiment, there was no great difference in transfer when using P1 as opposed to using P2. Evidently, although P1 possesses the sequence available to interact with U3, P1 cannot interact with U3 in a way that greatly improves transfer as tRNAlys3.
Previously we designed a reconstituted system in vitro to simulate HIV-1 minus strand transfer. In a DNA-primed transfer reaction, donor RNA template D199 promoted a transfer efficiency of about 35% with acceptor RNA template A97h. Later, an approximately two-fold increase in transfer was observed when using donor template D520 under the same reaction conditions. This result demonstrated that donor RNA sequences 3' to the PBS influence transfer efficiency. Examination of the effect of homology on transfer and the acceptor RNase H cleavage profile suggested that both D199 and D520 promoted transfer mainly through an acceptor invasion driven pathway. In this pathway an initial acceptor-cDNA contact at an invasion site spreads to complete terminus transfer of the cDNA. However, by comparing the significance of each transfer step–acceptor invasion, hybrid propagation, and terminal transfer-involved in these two donor systems, we presented evidence that for D520, efficient acceptor invasion increased the local concentration of the region of the acceptor complementary to the cDNA primer terminus, stimulating more efficient transfer through an acceptor proximity effect (23, 24).
The facts that a nonanucleotide segment in the Mal strain of HIV-1 could interact with tRNAlys3 to stimulate minus strand transfer and that this nonanucleotide segment is conserved throughout different strains of the HIV-1 genome suggest that the segment has an important role (56). These observations prompted us to examine whether introduction of the tRNAlys3-U3 interaction could overcome defects involved with inefficient invasion-driven transfer from D199, leading to more efficient overall transfer. Also, we wanted to test whether the tRNAlys3-U3 interaction would promote transfer of D520 to an even higher efficiency. The substrates utilized by Marquet and co-workers included a donor template, which represented the first 311 nt of the 5' terminus of viral genome, and an acceptor template, which represented the first 639 nt of the 3' terminus of viral genome (56). This transfer system allowed acceptor invasion driven pathway and tRNAlys3-U3 interaction. However, the donor template used in this system was 209 nt shorter at the 3' end than the donor template D520 used in our study. The tRNAlys3 primed transfer system of D520 would give us a model in which the stimulatory effect on transfer conferred by a long genomic sequence 3' of PBS (23) was also included.
To examine the effect of the tRNAlys3-U3 interaction on the minus strand transfer reaction of D199 and D520, we prepared A97h(-54), an acceptor not only capable of facilitating invasion driven transfer similar to A97h but also capable of interacting with tRNAlys3 by means of its nonanucleotide sequence complementarity. In tRNAlys3 primed reactions, with both donors, a significant stimulation in the transfer reaction was observed using A97h(-54) compared to using A97h. Differences in the transfer efficiency between the two donors were reduced when using A97h(-54) compared to using A97h. Also, the specific absence of tRNAlys3-U3 interaction in the longer acceptor, achieved by using either Δ-A97h(-54) or mut-A97h(-54), led to a significant decrease in transfer for both of the donor systems, an observation consistent with the mutational study performed by the Marquet group (56). We interpret these results to mean that the tRNAlys3-U3 interaction compensated for the inefficient acceptor invasion driven pathway exhibited by D199, greatly facilitating transfer to 78% by increasing the local concentration of acceptor, and further promoted D520 to reach a higher transfer efficiency of 85%.
tRNAlys3 primed D199 promoted transfer at an efficiency of 18% with A97h, while this number increased greatly to 78% with A97h(-54). Could this have meant that tRNAlys3-U3 interaction is sufficient to achieve maximal stimulation? To answer this question meaningfully, we measured the stimulatory ability of the tRNAlys3-U3 interaction with a substrate that does not support invasion-driven transfer, A19h(-54). For both D520 and D199, a decrease in transfer was observed when utilizing A19h(-54) compared to utilizing A97h(-54). Such a decrease was more significant with D199. Evidently, whether or not the acceptor invasion reaction is occurring with high efficiency, the proximity effect through tRNAlys3-U3 interaction needs the assistance of acceptor invasion occurring early on in the transfer reaction to achieve maximal stimulation.
To examine whether an alternative interaction of tRNAlys3-U3 with sequence complementarity at different positions could also promote transfer as efficiently as the tRNAlys3-U3 interaction identified by the Marquet group (56), acceptor template mat-A97h(-54) was generated. The fact that mat-A97h(-54) promoted transfer just as efficiently as A97h(-54) indicates that the tRNAlys3-U3 interaction, which facilitates more efficient transfer, is not restricted to the wild type viral sequence, but could be achieved through complementary mutations. Also, to analyze whether just sequence complementary between the primer and U3 in the acceptor is sufficient to stimulate minus strand transfer, or whether proper folding of tRNAlys3 is also essential for the reaction, the DNA primer P1 was designed. The potential P1–U3 interaction did not improve the transfer reaction of D199, suggesting that proper folding of the primer is required for effective primer-U3 interaction.
A noteworthy point is that DNA primer P2 is only complementary to the PBS in the viral genome, and is the same primer utilized to initiate minus strand DNA synthesis in our previous studies (23, 24). In the P2-primed transfer reaction with A97h, transfer efficiency was about 75% for D520 and 35% for D199 (Fig. 3E). In the tRNAlys3 primed transfer reaction with A97h, however, transfer efficiency was only about 40% for D520 and 18% for D199 (Fig. 1C). Clearly, the DNA primer P2 promoted more efficient transfer than the tRNAlys3 primer. One possible explanation is that in the absence of tRNAlys3-U3 interaction with A97h, the tRNAlys3 primer is in a conformation that is not entirely favorable for acceptor invasion initiated transfer.
Previously, when using acceptors of a bipartite structure which allowed acceptor invasion and terminal transfer but not hybrid propagation, we observed that blockage of hybrid propagation did not have a large effect on the transfer reaction with D520. We interpreted this result to mean that efficient acceptor invasion in the D520 system raises the local concentration of the 5' terminus of acceptor complementary to the cDNA 3' terminus and promotes transfer via an acceptor proximity effect. The results imply that invasion can stimulate terminus transfers by two parallel acting mechanisms. It would promote branch migration of the RNA-DNA hybrid formed at the invasion site until the hybrid expanded to the cDNA terminus to complete transfer. Alternatively, the binding of the acceptor at the invasion site would promote terminus transfer by proximity (24). In this study, the stimulation of transfer observed with the tRNAlys3-U3 interaction indicates that tRNAlys3-U3 interaction brings acceptor and elongated primer into proximity. In other previous work, a stimulation of transfer was also observed in a substrate system in which both the donor and acceptor RNAs harbored the functional motif dimerization initiation site (DIS). DIS sites were shown to adhere the two templates (63–65), again raising local concentration. The interaction was proposed to form the basis of the higher transfer efficiency (66, 67). Overall, these observations suggest that an acceptor proximity effect can be utilized by HIV-1 in several ways to promote transfer. Proximity can be achieved by either a tRNA-template interaction, a template-cDNA invasion (24), or a template-template hairpin-kissing process as in DIS (24, 66, 67).
Based on our current results, we propose that multiple mechanisms work together in successive steps to achieve efficient minus strand transfer of HIV-1 (Fig. 7). The reaction is initiated by a tRNAlys3 primer annealed to the PBS (68, 69). The sequence complementarity between tRNAlys3 and U3 in acceptor brings the acceptor 5' terminus closer to primer-donor complex. As synthesis proceeds, the acceptor invades the synthesized cDNA in the internal region of R, facilitating the acceptor 3' terminus to anneal to the elongated primer-donor complex at the invasion site. The acceptor-cDNA hybrid propagates, by branch migration or proximity interactions, from the invasion site to the 5' terminus of acceptor until transfer is completed (20,21,23,24). Presumably, this complex, multi-step process has evolved because highly efficient −sssDNA transfer has been critical for the long term survival of HIV-1.
We thank Dr. Dorota Piekna, Jason Stewart, Sean Rigby, and Wen Shen for helpful discussions and critical reading of the manuscript.
This work was supported by National Institutes of Health Grant GM049573 (to R. A. B.).