Here, we took advantage of a viable mutant allele of the essential IMP3
gene that permits cell growth to investigate the Imp3p functions. The mutant IMPQ strain harboured relevant growth and functional defects. The IMPQ strain appeared to be cold sensitive and its growth was lightly slowed compared to the wild-type strain. Both phenotypes were already reported for mutations affecting ribosome assembly, or pre-rRNA processing 
, notably with a Mpp10p mutant 
. As expected, the IMPQ strain was impaired for ribosome biogenesis, presenting a decrease of free 40S ribosomal subunits and less mature 18S rRNA. Results also suggested Imp3Qp interacted less efficiently with Mpp10p, reducing the protein import into the nucleus, which could indeed produce the observed ribosome biogenesis defects.
We then investigated the impact of the mutation on protein translation accuracy. No effect was detected on stop codon readthrough and −1 frameshifting, but +1 frameshifting was significantly increased. The effect was observed with two programmed +1 frameshift sequences using either a rare sense codon (EST3
) or a shifty stop codon (OAZ1
) as a stimulatory element 
, and are thus controlled by different mechanisms. This indicates that the effect likely concerned a common mechanism. Since for both sequences, stimulatory elements in the A site of the ribosome induce pausing, one may suggest the ribosome pausing time is increased in presence of Imp3Qp, due to some expected ribosomal alterations occurring in the strain, as previously inferred for −1 frameshift signals and delayed rRNA processing 
. Actually, the pre-rRNA cleavage defects might produce aberrant 40S small subunits altered in frame maintenance. Yet, we cannot exclude an unknown function of the Imp3 protein being impaired in the IMPQ strain. The IMPQ strain also appeared to be highly sensitive to G418 and paromomycin, two aminoglycosides targeting the decoding center of the 18S rRNA in the small subunit. However, the IMPQ strain was not sensitive to the other tested aminoglycosides, the effect thus seemed to be specific to a subclass of aminoglycosides.
The IMPQ strain was also highly sensitive to bleomycin and to the structurally related phleomycin, which produce different kinds of toxic DNA lesions 
. Sensitivity to an elevated concentration of camptothecin, HU and MMS, but not to H2
was revealed. All phenotypes suggest an involvement of Imp3p in double strand break (DSB) repair. To date the Imp3 protein has never been observed participating in an extra-ribosomal biogenesis pathway. However, recently, Imp4p partner of Imp3p in the Mpp10 subcomplex of the SSU processome was related to DNA DSB repair 
and telomere length control 
. This and the present work highlighted the putative role of the whole Mpp10 subcomplex in DNA repair processes.
Furthermore, a recent genome-scale study of synthetic genetic interactions in yeast revealed a connection between Mpp10p and several proteins implicated in telomere maintenance and silencing (Stm1p, Upf3p, Nmd2p, Dpb4p) 
. Meaningfully, each partner of the Imp3-Mpp10-Imp4 subcomplex was independently, and by different approaches, linked to the telomere metabolism. Nevertheless, we can not exclude indirect effects due to reduced translation and further investigations are needed to elucidate this issue.
An indirect role of Imp3p in telomere length control was anticipated through the increase of the EST3
+1 frameshifting, which produces more functional subunits of the telomerase. Although western blot analysis confirmed that the Est3 protein was slightly (~10%) more abundant in the IMPQ strain compared to the FS1 strain (data not shown), it could not be responsible for the telomere length difference. Yet, telomeres were still longer in the IMPQ strain deleted for the EST3
gene. We also tested a strain deleted for Upf1p (“Up Frameshift 1 protein”), a helicase implicated in non-sense mediated mRNA decay, which displays an increased shifty stop-dependent +1 frameshifting 
. The strain was observed to be resistant to phleomycin (data not shown) and is described for having telomere shortening 
. Results thus strongly argued for a role of Imp3p in telomere metabolism and antibiotic sensitivity independently of the telomerase or increased +1 frameshifting activities.
Yeast telomeres are transcribed into telomeric repeat-containing RNA (TERRA) 
, which inhibit telomerase action via
a DNA/RNA hybrid 
. We could infer a role of Imp3p, or a larger snoRNP particle, in processing of these TERRA molecules, or of the TLC1
The role of Imp3p in pre-rRNA processing is evolutionary conserved between yeast and mammals. It might therefore be interesting to inquire about functions of Imp3 proteins in DNA repair and telomere metabolism in higher organisms. It might be a general property of RNA-processing proteins to have dual functions in the nucleus.