EFFECTS OF DELETION MUTATIONS IN THE YEAST CES1 PROTEIN ON CELL-GROWTH AND MORPHOLOGY AND ON HIGH COPY SUPPRESSION OF MUTATIONS IN MESSENGER-RNA CAPPING ENZYME AND TRANSLATION INITIATION-FACTOR 4A

The homologous Saccharomyces cerevisiae genes CES1 and CES4 act as hig h copy suppressors of temperature-sensitive mutations of Ceg1p, the ye ast mRNA capping enzyme. Neither CES1 nor CES4 is essential for cell g rowth. We find that a double deletion mutant (Delta ces1 Delta ces4) g rows at 25-37 degrees C, but not at 16 degrees C. Delta ces1 Delta ces 4 cells display gross defects in cell shape and budding even at permis sive temperatures. Functional analysis of CES1 deletion mutants define s a 145 amino acid C-terminal segment of the 915 amino acid Ces1 prote in that is necessary and sufficient to complement the Delta ces1 Delta ces4 cold-sensitive phenotype, to restore normal morphology and to su ppress the temparature-sensitive mutant ceg1-25. A 147 amino acid C-te rminal segment of the 942 amino acid Ces4 protein is sufficient to car ry out these same functions. Within this carboxyl domain Ces1p and Ces 4p are 80% identical to one another. We report isolation of CES1 in a separate screen for high copy suppression of a temperature-sensitive m utation (A79V) of the yeast translation initiation factor Tif1p (elF-4 A). Deletion of the N-terminal 249 amino acids of Ces1p abolished tif1 -A79V suppressor function. CES4 on a multicopy plasmid was unable to s uppress tif1-A79V. We surmise that whereas the carboxyl domains of Ces 1p and Ces4p are functionally redundant in controlling cell morphology and in suppressing ceg1-25, full-length Ces1p and Ces4p evince distin ct genetic interactions that are likely mediated by their N-terminal s egments.