Escherichia coli purB encodes adenylosuccinate lyase (ASL), the enzyme that catalyzes step 8 in the pathway for de novo synthesis of IMP and also the final reaction in the two-step sequence from IMP to AMP. Gene purB was cloned and found to encode an ASL protein of 435 amino acids having a calculated molecular weight of 49,225. E. coli ASL is homologous to the corresponding enzymes from Bacillus subtilis and chickens and also to fumarase from B. subtilis. Gene phoP is 232 bp downstream of purB. Gene purB is regulated threefold by the purine pool and purR. Transcriptional regulation of purB involves binding of the purine repressor to the 16-bp conserved pur regulon operator. The purB operator is 224 bp downstream of the transcription start site and overlaps codons 62 to 67 in the protein-coding sequence.
Here, the crystal structure of adenylosuccinate lyase from Escherichia coli was determined to 1.9 Å resolution.
Adenylosuccinate lyase (ASL) is an enzyme from the purine-biosynthetic pathway that catalyzes the cleavage of 5-aminoimidazole-4-(N-succinylcarboxamide) ribonucleotide (SAICAR) to 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and fumarate. ASL is also responsible for the conversion of succinyladenosine monophosphate (SAMP) to adenosine monophosphate (AMP) and fumarate. Here, the crystal structure of adenylosuccinate lyase from Escherichia coli was determined to 1.9 Å resolution. The enzyme adopts a substrate-bound conformation as a result of the presence of two phosphate ions bound in the active site. Comparison with previously solved structures of the apoenzyme and an SAMP-bound H171A mutant reveals a conformational change at His171 associated with substrate binding and confirms the role of this residue as a catalytic acid.
adenylosuccinate lyase; ASL; PurB; purine biosynthesis
Purine biosynthesis requires ten enzymatic transformations to generate inosine monophosphate. PurF, PurD, PurL, PurM, PurC, and PurB are common to all pathways, while PurN or PurT, PurK/PurE-I or PurE-II, PurH or PurP, and PurJ or PurO catalyze the same steps in different organisms. X-ray crystal structures are available for all 15 purine biosynthetic enzymes, including seven ATP-dependent enzymes, two amidotransferases and two tetrahydrofolate-dependent enzymes. Here we summarize the structures of the purine biosynthetic enzymes, discuss similarities and differences, and present arguments for pathway evolution. Four of the ATP-dependent enzymes belong to the ATP-grasp superfamily and two to the PurM superfamily. The amidotransferases are unrelated with one utilizing an NTN-glutaminase and the other utilizing a triad glutaminase. Likewise the tetrahydrofolate-dependent enzymes are unrelated. Ancestral proteins may have included a broad specificity enzyme instead of PurD, PurT, PurK, PurC, and PurP, and a separate enzyme instead of PurM and PurL.
purine biosynthesis; protein evolution; ATP-grasp superfamily; PurM superfamily; amidotransferases
Fusions of lacZ were constructed to genes in each of the loci involved in de novo synthesis of IMP. The expression of each pur-lacZ fusion was determined in isogenic purR and purR+ strains. These measurements indicated 5- to 17-fold coregulation of genes purF, purHD, purC, purMN, purL, and purEK and thus confirm the existence of a pur regulon. Gene purB, which encodes an enzyme involved in synthesis of IMP and in the AMP branch of the pathway, was not regulated by purR. Each locus of the pur regulon contains a 16-base-pair conserved operator sequence that overlaps with the promoter. The purR product, purine repressor, was shown to bind specifically to each operator. Thus, binding of repressor to each operator of pur regulon genes negatively coregulates expression.
Genome sequencing projects on two relapsing fever spirochetes, Borrelia hermsii and Borrelia turicatae, revealed differences in genes involved in purine metabolism and salvage compared to those in the Lyme disease spirochete Borrelia burgdorferi. The relapsing fever spirochetes contained six open reading frames that are absent from the B. burgdorferi genome. These genes included those for hypoxanthine-guanine phosphoribosyltransferase (hpt), adenylosuccinate synthase (purA), adenylosuccinate lyase (purB), auxiliary protein (nrdI), the ribonucleotide-diphosphate reductase alpha subunit (nrdE), and the ribonucleotide-diphosphate reductase beta subunit (nrdF). Southern blot assays with multiple Borrelia species and isolates confirmed the presence of these genes in the relapsing fever group of spirochetes but not in B. burgdorferi and related species. TaqMan real-time reverse transcription-PCR demonstrated that the chromosomal genes (hpt, purA, and purB) were transcribed in vitro and in mice. Phosphoribosyltransferase assays revealed that, in general, B. hermsii exhibited significantly higher activity than did the B. burgdorferi cell lysate, and enzymatic activity was observed with adenine, hypoxanthine, and guanine as substrates. B. burgdorferi showed low but detectable phosphoribosyltransferase activity with hypoxanthine even though the genome lacks a discernible ortholog to the hpt gene in the relapsing fever spirochetes. B. hermsii incorporated radiolabeled hypoxanthine into RNA and DNA to a much greater extent than did B. burgdorferi. This complete pathway for purine salvage in the relapsing fever spirochetes may contribute, in part, to these spirochetes achieving high cell densities in blood.
Escherichia coli purA encodes adenylosuccinate synthetase, one of two enzymes required for synthesis of AMP from IMP. purA is subject to two- to threefold regulation by purR and about twofold regulation by a purR-independent mechanism. The 5'-flanking region of purA confers purR-dependent transcriptional regulation of purA but not the purR-independent regulation. Two operator sites in the 5'-flanking region which bind purine repressor in vitro and are required for in vivo regulation were identified. The purR-independent regulation may be posttranscriptional. It is now established that all transcription units involved in de novo synthesis of purine nucleotides, nine pur operons, as well as purR itself and guaBA, are subject to purR control.
Purine biosynthesis is considered to be a promising new antibiotic target; in this article, the structural and biochemical characterization of S. aureus PurK and PurE are described.
With the rapid rise of methicillin-resistant Staphylococcus aureus infections, new strategies against S. aureus are urgently needed. De novo purine biosynthesis is a promising yet unexploited target, insofar as abundant evidence has shown that bacteria with compromised purine biosynthesis are attenuated. Fundamental differences exist within the process by which humans and bacteria convert 5-aminoimidazole ribonucleotide (AIR) to 4-carboxy-5-aminoimidazole ribonucleotide (CAIR). In bacteria, this transformation occurs through a two-step conversion catalyzed by PurK and PurE; in humans, it is mediated by a one-step conversion catalyzed by class II PurE. Thus, these bacterial enzymes are potential targets for selective antibiotic development. Here, the first comprehensive structural and biochemical characterization of PurK and PurE from S. aureus is presented. Structural analysis of S. aureus PurK reveals a nonconserved phenylalanine near the AIR-binding site that occupies the putative position of the imidazole ring of AIR. Mutation of this phenylalanine to isoleucine or tryptophan reduced the enzyme efficiency by around tenfold. The K
m for bicarbonate was determined for the first time for a PurK enzyme and was found to be ∼18.8 mM. The structure of PurE is described in comparison to that of human class II PurE. It is confirmed biochemically that His38 is essential for function. These studies aim to provide foundations for future structure-based drug-discovery efforts against S. aureus purine biosynthesis.
purine biosynthesis; Staphylococcus aureus; enzyme kinetics; structure-based drug discovery; antimicrobials
ATP participates in many cellular metabolic processes as a major substrate to supply energy. Many systems for acidic resistance (AR) under extremely acidic conditions have been reported, but the role of ATP has not been examined. To clarify whether or not ATP is necessary for the AR in Escherichia coli, the AR of mutants deficient in genes for ATP biosynthesis was investigated in this study. The deletion of purA or purB, each of which encodes enzymes to produce AMP from inosinate (IMP), markedly decreased the AR. The content of ATP in these mutants decreased rapidly at pH 2.5 compared to that of the wild type. The AR was again decreased significantly by the mutation of adk, which encoded an enzyme to produce ADP from AMP. The DNA damage in the purA and purB mutants was higher than that in the wild type. These results demonstrated that metabolic processes that require ATP participate in survival under extremely acidic conditions, and that one such system is the ATP-dependent DNA repair system.
A point mutation (E115K) resulting in slower growth of Escherichia coli DH5α and XL1-Blue in minimal media was identified in the purB gene, coding for adenylosuccinate lyase (ASL), through complementation with an E. coli K-12 genomic library and serial subcultures. Chromosomal modification reversing the mutation to the wild type restored growth phenotypes in minimal media.
Controversy exists as to whether the purine nucleotide cycle is important in normal skeletal muscle function. Patients with disruption of the cycle from a deficiency of AMP deaminase exhibit variable degrees of muscle dysfunction. An animal model was used to examine the effect of inhibition of the purine nucleotide cycle on muscle function. When the compound 5-amino-4-imidazolecarboxamide riboside (AICAriboside) is phosphorylated to the riboside monophosphate in the myocyte it is an inhibitor of adenylosuccinate lyase, one of the enzymes of the purine nucleotide cycle. AICAriboside was infused in 28 mice, and 22 mice received saline. Gastrocnemius muscle function was assessed in situ by recording isometric tension developed during stimulation. The purine nucleotide content of the muscle was measured before and after stimulation. Disruption of the purine nucleotide cycle during muscle stimulation was evidenced by a greater accumulation of adenylosuccinate, the substrate for adenylosuccinate lyase, in the animals receiving AICAriboside (0.60 +/- 0.10 vs. 0.05 +/- 0.01 nmol/mumol total creatine, P less than 0.0001). There was also a larger accumulation of inosine monophosphate in the AICAriboside vs. saline-treated animals at end stimulation (73 +/- 6 vs. 56 +/- 5 nmol/mumol total creatine, P less than 0.03). Inhibition of flux through the cycle was accompanied by muscle dysfunction during stimulation. Total developed tension in the AICAriboside group was 40% less than in the saline group (3,023 +/- 1,170 vs. 5,090 +/- 450 g . s, P less than 0.002). An index of energy production can be obtained by comparing the change in total phosphagen content per unit of developed tension in the two groups. This index indicates that less high energy phosphate compounds were generated in the AICAriboside group, suggesting that interruption of the purine nucleotide cycle interfered with energy production in the muscle. We conclude from these studies that defective energy generation is one mechanism whereby disruption of the purine nucleotide cycle produces muscle dysfunction.
The function of the Staphylococcus aureus eukaryotic-like serine/threonine protein kinase PknB was investigated by performing transcriptome analysis using DNA microarray technology and biochemical assays. The transcriptional profile revealed a strong regulatory impact of PknB on the expression of genes encoding proteins which are involved in purine and pyrimidine biosynthesis, cell wall metabolism, autolysis, and glutamine synthesis. Functional activity of overexpressed and purified PknB kinase was demonstrated using the myelin basic protein as a surrogate substrate. Phosphorylation occurred in a time-dependent manner with Mn2+ as a preferred cofactor. Furthermore, biochemical characterization revealed regulation of adenylosuccinate synthase (PurA) activity by phosphorylation. Phosphorylated PurA showed a 1.8-fold decrease in enzymatic activity compared to unphosphorylated PurA. Loss of PknB led to formation of larger cell clusters, and a pknB deletion strain showed 32-fold-higher sensitivity to the cell wall-active antibiotic tunicamycin. The results of this study strongly indicate that PknB has a role in regulation of purine biosynthesis, autolysis, and central metabolic processes in S. aureus.
In a previous study, transformation demonstrated that a gene governing enterotoxin A production (entA+) in Staphylococcus aureus strain S-6 was located on the chromosome between the purB110 and ilv-129 markers; in contrast, the entA+ gene of strain FRI-196E was shown not to be located in the same position. In the current study, 54 enterotoxin A-producing strains of S. aureus were examined to locate the entA+ gene. Conventional transformation procedures and a series of multiply marked derivatives of NCTC 8325 were used as recipients for chromosomal mapping. Of the 54 strains tested, 23 were found to contain the entA+ gene at the original locus between the purB110 and ilv-129 markers. Twenty-seven strains could not be analyzed either because their DNA was genetically ineffective in transforming strain 8325 (23 strains), or Pur+ Ilv+ transformants could not be recovered (four strains). Four other strains contained an entA+ gene that could not be located in any of the chromosomal linkage groups. A new insertion site for Tn551 was located within the hla+ gene involved in alpha-toxin production. It eliminated alpha-toxin production and was used to separate the entA+ gene from the hla+ marker in the purB110-ilv-129 region. This segment of the chromosome is shown to consist of the purB110, entA+, hla+, and ilv-129 markers in that order.
The crystal structure of N
5-carboxyaminoimidazole ribonucleotide synthase from Bacillus anthracis with only an Mg cation provides some insight into the catalytic mechanism of this enzyme and the role of a crucial loop during catalysis.
The apo structure of N
5-carboxyaminoimidazole ribonucleotide synthase (PurK) from Bacillus anthracis (baPurK) with Mg2+ in the active site is reported at 1.96 Å resolution. PurK is an enzyme in the purine-biosynthetic pathway, unique to prokaryotes, that converts 5-aminoimidazole ribonucleotide to N
5-carboxyaminoimidazole ribonucleotide and has been suggested as a potential antimicrobial drug target. Two interesting features of baPurK are a flexible B-loop (residues 149/150–157) that is in close contact with the active site and the binding of Mg2+ to the active site without additional ligands.
N5-carboxyaminoimidazole ribonucleotide synthase; PurK; Bacillus anthracis; purine biosynthesis
We isolated a strain of Escherichia coli K-12 in which the lac structural genes are fused to the purB control region and used this strain to study the regulation of the purA and purB loci. The purA locus was derepressed in response to either limiting adenine or guanine growth conditions in the presence of excess guanine or adenine, respectively. The presence of hypoxanthine in the culture medium did not have any effect on the expression of the purA locus. The purB locus responded to limiting adenine growth conditions in the presence of either excess hypoxanthine or guanine alone but not when both hypoxanthine and guanine were present.
The purB and purH mutants of Mesorhizobium loti exhibited purine auxotrophy and nodulation deficiency on Lotus japonicus. In the presence of adenine, only the purH mutant induced nodule formation and the purB mutant produced few infection threads, suggesting that 5-aminoimidazole-4-carboxamide ribonucleotide biosynthesis catalyzed by PurB is required for the establishment of symbiosis.
Previous microarray data (E. Mongodin, J. Finan, M. W. Climo, A. Rosato, S. Gill, and G. L. Archer, J. Bacteriol. 185:4638-4643, 2003) noted an association in two vancomycin-intermediate Staphylococcus aureus (VISA) strains between high-level, passage-induced vancomycin resistance, a marked increase in the transcription of purine biosynthetic genes, and mutation of the putative purine regulator purR. Initial studies to report on the possible association between vancomycin resistance and alterations in purine metabolism in one of these strains (VP-32) confirmed, by Western analysis, an increase in the translation of PurH and PurM, two purine pathway enzymes. In addition, PurR was identified, by knockout and complementation in a vancomycin-susceptible strain, as a repressor of the purine biosynthetic operon in S. aureus, and the PurR missense mutation was shown to inactivate the repressor. However, despite the apparent relationship between increased purine biosynthesis and increased vancomycin resistance in VP-32, neither the addition of exogenous purines to a defined growth medium nor the truncation or inactivation of purR improved the growth of vancomycin-susceptible S. aureus in the presence of vancomycin. Furthermore, the passage of additional vancomycin-susceptible and VISA strains to high-level vancomycin resistance occurred without changes in cellular purine metabolism or mutation of purR despite the development of thickened cell walls in passaged strains. Thus, we could confirm neither a role for altered purine metabolism in the development of vancomycin resistance nor its requirement for the maintenance of a thickened cell wall. The failure of biochemical and physiological studies to support the association between transcription and phenotype initially found in careful microarray studies emphasizes the importance of follow-up investigations to confirm microarray observations.
Bacillus subtilis genes purA, encoding adenylosuccinate synthetase, and guaA, coding for GMP synthetase, appear to be lethal when cloned in multicopy plasmids in Escherichia coli. The nucleotide sequences of purA and guaA were determined from a series of gene fragments isolated by polymerase chain reaction amplification, library screening, and plasmid rescue techniques. Identifications were based on amino acid sequence alignments with enzymes from other organisms. Comparison of the 5'-flanking regions of purA and guaA with the pur operon suggests similarities in mechanisms for gene regulation. Nucleotide sequences are now available for all genes involved in the 14-step pathway for de novo purine nucleotide synthesis in B. subtilis.
Mutations in Salmonella typhimurium strains lacking nonspecific acid phosphatase mapped in two unlinked loci. One of these, phoP, was cotransducible by phage P22 with purB, whereas the second, phoN, was cotransducible by phage P1 with purA. Mutants with temperature-sensitive nonspecific acid phosphatase activity (measured in whole cells) were also isolated. A phoN mutant with thermolabile whole-cell activity was isolated directly from wild-type LT-2. Several other mutants with temperature-sensitive enzyme activity were also isolated as revertants of phoN mutants. These data suggest that phoN might be a structural locus for nonspecific acid phosphatase. The observation that a mutation resulting in high level of nonspecific acid phosphatase mapped in phoP suggests a possible regulatory role for this locus.
Particle-free extracts prepared from kidney cortex of rat catalyze the formation of ammonia via the purine nucleotide cycle. The cycle generates ammonia and fumarate from aspartate, using catalytic amounts of inosine monophosphate, adenylosuccinate, and adenosine monophosphate. The specific activities of the enzymes of the cycle are 1.27+/-0.27 nmol/mg protein per min (SE) for adenoylosuccinate synthetase, 1.38+/-0.16 for adenylosuccinase, and 44.0+/-3.3 for AMP deaminase. Compared with controls, extracts prepared from kidneys of rats fed ammonium chloride for 2 days show a 60% increase in adenylosuccinate synthetase and a threefold increase in adenylosuccinase activity, and a greater and more rapid synthesis of ammonia and adenine nucleotide from aspartate and inosine monophosphate. Extracts prepared from kidneys of rats fed a potassium-deficient diet show a twofold increase in adenylosuccinate synthetase and a threefold increase in adenylosuccinase activity. In such extracts the rate of synthesis of ammonia and adenine nucleotide from aspartate and inosine monophosphate is also increased. These results show that the reactions of the purine nucleotide cycle are present and can operate in extracts of kidney cortex. The operational capacity of the cycle is accelerated by ammonium chloride feeding and potassium depletion, conditions known to increase renal ammonia excretion. Extracts of kidney cortex convert inosine monophosphate to uric acid. This is prevented by addition of allopurinol of 1-pyrophosphoryl ribose 5-phosphate to the reaction mixture.
Inosine 5′-monophosphate (IMP) cyclohydrolase catalyzes the cyclization of 5-formaminoimidazole-4-carboxamide ribonucleotide (FAICAR) to IMP in the final step of de novo purine biosynthesis. Two major types of this enzyme have been discovered to date: PurH in Bacteria and Eukarya, and PurO in Archaea. The structure of the MTH1020 gene product from Methanothermobacter thermoautotrophicus was previously solved without functional annotation but shows high amino acid sequence similarity to other PurOs. We determined the crystal structure of the MTH1020 gene product in complex with either IMP or 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) at 2.0 Å and 2.6 Å resolution, respectively. Based on the sequence analysis, ligand-bound structures, and biochemical data, MTH1020 is confirmed as an archaeal IMP cyclohydrolase, thus designated as MthPurO. MthPurO has a four-layered αββα core structure, showing an N-terminal nucleophile (NTN) hydrolase fold. The active site is located at the deep pocket between two central β-sheets and contains residues strictly conserved within PurOs. Comparisons of the two types of IMP cyclohydrolase, PurO and PurH, revealed that there are no similarities in sequence, structure, or the active site architecture, suggesting that they are evolutionarily not related to each other. The MjR31K mutant of PurO from Methanocaldococcus jannaschii showed 76% decreased activity and MjE102Q mutation completely abolished enzymatic activity, suggesting that these highly conserved residues play critical roles in catalysis. Interestingly, green fluorescent protein (GFP), which has no structural homology to either PurO or PurH but catalyzes a similar intramolecular cyclohydrolase reaction required for chromophore maturation, utilizes Arg96 and Glu222 in a mechanism analogous to that of PurO.
Production of alpha-toxin (the Hla+ phenotype, controlled by the Hla gene and scored as alpha-hemolytic activity) is a property of some isolates of Staphylococcus aureus NCTC 8325 and not of others. Genetic transformation between strains differing in the Hla phenotype revealed that the hla+ gene resides in the following sequence: purB110-bla+-hla+-ilv-129-pig-131; previously, the enterotoxin A (entA) gene of strain S-6 was shown to map very close to hla+. The hla+ mutations occurring naturally in strain Ps6 and after various mutagenic treatments in strains 8325 and 233 also mapped between bla+ and ilv-129. Among the isolates of strain 8325, the Hla+ phenotype was always associated with fibrinolytic activity, whereas Hla- isolates were non-fibrinolytic. This relationship was also observed among transformants selected for their Hla+ or Hla- phenotypes. The failure of Hla- strains and mutants to revert to hla+ at detectable frequencies, the instability of the Hla+ phenotype, and the previously observed pattern of recombination of the hla+ and entA+ determinants lend support to the view that hla+ may reside on a transposon; according to this view, Hla- mutants have lost the hla+-bearing transposon. It remains unclear whether hla+ is the structural gene for alpha-toxin.
The inhibition of Escherichia coli strain B and strain W-11 by 6-methylpurine depended on the formation of 6-methylpurine ribonucleotide by the action of adenine phosphoribosyltransferase (AMP: pyrophosphate phosphoribosyltransferase, EC 220.127.116.11). 6-Methylpurine ribonucleotide inhibited the de novo synthesis of purines, presumably via pseudofeedback inhibition of phosphoribosylpyrophosphate amidotransferase (EC 18.104.22.168). The same mechanism accounted for its inhibition of adenylosuccinate synthetase [IMP: l-aspartate ligase (GDP), EC 22.214.171.124]. Adenine and 6-methylaminopurine prevented inhibition by competing for the action of adenine phosphoribosyltransferase. In addition, adenine reversed this inhibition by replenishing the AMP to bypass both sites of inhibition. Nonproliferating suspensions of strain B-94, which lacked adenylosuccinate lyase (EC 126.96.36.199), converted exogenous hypoxanthine and aspartate to succinoadenine derivatives which accumulated in the medium. Compounds which inhibited adenylosuccinate synthetase inhibited accumulation of the succinoadenine derivatives. A method was described for the isolation of mutants which potentially possessed an altered adenylosuccinate synthetase.
The major pathways of ribonucleotide biosynthesis in Mycoplasma mycoides subsp. mycoides were proposed previously from studies of its usage of radioactive purines and pyrimidines. To interpret more fully the pattern of purine usage, we have assayed cell-free extracts of this organism for several enzymes associated with the salvage synthesis of purine nucleotides. M. mycoides possessed phosphoribosyltransferases for adenine, guanine, and hypoxanthine, purine nucleoside phosphorylase, GMP reductase, GMP kinase, adenylosuccinate synthetase, and adenylosuccinate lyase. Purine nucleoside kinase and adenosine deaminase were not detected. Examination of kinetic properties and regulation of some of the above enzymes revealed differences between M. mycoides and Escherichia coli. Most notable of these were the greater susceptibility of the enzymes from M. mycoides to inhibition by nucleotides and the more widespread involvement of GMP as an inhibitor. Observations on enzyme activities in vitro allow an adequate explanation of the capacity of guanine to provide M. mycoides with its full requirement for purine nucleotides.
The structure of PurL from T. thermophilus HB8 in complex with an adenine nucleotide is reported at 2.35 Å resolution.
The crystal structure of PurL from Thermus thermophilus HB8 (TtPurL; TTHA1519) was determined in complex with an adenine nucleotide, PO4
3− and Mg2+ at 2.35 Å resolution. TtPurL consists of 29 α-helices and 28 β-strands, and one loop is disordered. TtPurL consists of four domains, A1, A2, B1 and B2, and the structures of the A1–B1 and A2–B2 domains were almost identical to each other. Although the sequence identity between TtPurL and PurL from Thermotoga maritima (TmPurL) is higher than that between TtPurL and the PurL domain of the large PurL from Salmonella typhimurium (StPurL), the secondary structure of TtPurL is much more similar to that of StPurL than to that of TmPurL.
amidotransferases; purine nucleotide biosynthetic pathway
Escherichia coli purB is regulated by a repressor-operator interaction. The purB operator is 242 bp downstream from the transcription start site and overlaps condons 62 to 67 in the protein-coding sequence (B. He, J. M. Smith, and H. Zalkin, J. Bacteriol. 174:130-136, 1992). The mechanism by which the repressor-operator interaction functions to repress transcription was investigated by a combination of promoter replacement experiments and RNA analyses. By using a trp promoter replacement that deleted 5' flanking DNA to position -986, purB expression was increased sevenfold, yet normal two- to threefold regulation was maintained. This indicates that repressor-operator control is independent of the purB promoter and other 5' flanking sequences. Transcriptional regulation was likewise independent of coupled translation. An approximately 260-nucleotide truncated in vivo purB mRNA was identified which was dependent upon repressor-operator interaction. Thus, binding of purine repressor to the purB operator inhibits transcription elongation by a roadblock mechanism. The roadblock was not influenced by a sevenfold increase in promoter strength or by an operator mutation resulting in a 2.5-fold increase in repressor-operator affinity.