SSL1, A SUPPRESSOR OF A HIS4 5'-UTR STEM LOOP MUTATION, IS ESSENTIAL FOR TRANSLATION INITIATION AND AFFECTS UV RESISTANCE IN YEAST

The SSL1 locus was identified as a trans-acting suppressor that restores HIS4 expression despite a stem-loop structure in the 5'-UTR. SSL1 encodes an essential protein of 52 kD with features characteristic of a protein with multiple zinc fingers. The mechanism of SSL1 suppression is not related to altering his4 transcription or removing the stem-loop sequence from the 5'-UTR; rather, 3- to 5-fold increases in His4 translational expression are observed indicating a post-transcriptional mechanism for SSL1 suppression. SSL1 suppressor mutants that are conditional for growth have altered polysome profiles at the restrictive temperature, and their cell-free extracts are thermolabile in their ability to translate exogenously added mRNA. In addition, the mechanism of suppression appears to be specific for stem-loop structures placed near the 5' end of the message as opposed to a stem-loop located at a downstream position in the 5'-UTR. These observations suggest a role for this protein in promoting translation initiation presumably at the level of ribosomal binding to mRNA. Surprisingly, SSL1 suppressor mutations that are shown to confer an in vivo and in vitro defect in translation initiation also rendered yeast hypersensitive to UV irradiation. This latter phenotype was observed previously with a mutation in the SSL2 suppressor gene, which encodes the yeast homolog of the human gene ERCC-3, for which a defective form causes xeroderma pigmentosum. In light of the related effects of mutations in the SSL1 and SSL2 genes, the encoded proteins may functionally interact both to promote DNA repair and perform an essential function during translation initiation.