RIBOSOMAL ACIDIC PHOSPHOPROTEINS P1 AND P2 ARE NOT REQUIRED FOR CELL VIABILITY BUT REGULATE THE PATTERN OF PROTEIN EXPRESSION IN SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae strains with either three inactivated genes ( triple disruptants) or four inactivated genes (quadruple disruptants) encoding the four acidic ribosomal phosphoproteins, YP1 alpha, YP1 bet a, YP2 alpha, and YP2 beta, present in this species have been obtained , Ribosomes from the triple disruptants and, obviously, those from the quadruple strain do not have bound P proteins, All disrupted strains are viable; however, they show a cold-sensitive phenotype, growing ver y poorly at 23 degrees C. Cell extracts from the quadruple-disruptant strain are about 30% as active as the control in protein synthesis ass ays and are stimulated by the addition of free acidic P proteins, Stra ins lacking acidic proteins do not have a higher suppressor activity t han the parental strains, and cell extracts derived from the quadruple disruptant do not show a higher degree of misreading, indicating that the absence of acidic proteins does not affect the accuracy of the ri bosomes, However, the patterns of protein expressed in the cells as we ll as in the cell-free protein system are affected by the absence of P proteins from the particles; a wild-type pattern is restored upon add ition of exogenous P proteins to the cell extract, In addition, strain s carrying P-protein-deficient ribosomes are unable to sporulate but r ecover this capacity upon transformation with one of the missing genes , These results indicate that acidic proteins are not an absolute requ irement for protein synthesis but regulate the activity of the 60S sub unit, affecting the translation of certain mRNAs differently.