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 te sted translational suppressors, including codon-specific tRNA suppress ors as well as codon-nonspecific omnipotent suppressors. The SAL6 gene has now been cloned by complementation of the increased suppression e fficiency and cold sensitivity caused by sal6-1 in the presence of the omnipotent suppressor sup45. Physical analysis maps SAL6 to chromosom e XVI between TPK2 and spt14. The SAL6 gene encodes avery basic 549-am ino 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-termina l extension found in SAL6, like those in the PPZ proteins, is serine-r ich. The salb-1 mutation is a frameshift at amino acid position 271 wh ich 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 s uppressor strain backgrounds. The role of the serine-rich N terminus i s unclear, since sal6 phenotypes are fully complemented by a SAL6 alle le that contains an in-frame deletion of most of this region. High cop y number plasmids containing wild-type SAL6 cause antisuppresser pheno types in suppressor strains. These results suggest that the accuracy o f protein synthesis is affected by the levels of phosphorylation of th e target(s) of SAL6.