The Yeast Translational Allosuppressor, SAL6: a New Member of the PP1-like Phosphatase Family with a Long Serine-rich N-terminal Extension

The allosuppressor mutation, sal6-1, enhances the efficiency of all tested translational suppressors, including codon-specific tRNA suppressors as well as codon-nonspecific omnipotent suppressors. The SAL6 gene has now been cloned by complementation of the increased suppression efficiency and cold sensitivity caused by sal6-1 in the presence of the omnipotent suppressor sup45. Physical analysis maps SAL6 to chromosome XVI between TPK2 and spt14. The SAL6 gene encodes a very basic 549-amino acid protein whose C-terminal catalytic region of 265 residues is 63% identical to serine/threonine PP1 phosphatases, and 66% identical to yeast PPZ1 and PPZ2 phosphatases. The unusual 235 residue N-terminal extension found in SAL6, like those in the PPZ proteins, is serine-rich. The sal6-1 mutation is a frameshift at amino acid position 271 which destroys the presumed phosphatase catalytic domain of the protein. Disruptions of the entire SAL6 gene are viable, cause a slight growth defect on glycerol medium, and produce allosuppressor phenotypes in suppressor strain backgrounds. The role of the serine-rich N terminus is unclear, since sal6 phenotypes are fully complemented by a SAL6 allele that contains an in-frame deletion of most of this region. High copy number plasmids containing wild-type SAL6 cause antisuppressor phenotypes in suppressor strains. These results suggest that the accuracy of protein synthesis is affected by the levels of phosphorylation of the target(s) of SAL6.