RIBOSOMAL-PROTEIN P0, CONTRARY TO PHOSPHOPROTEINS P1 AND P2, IS REQUIRED FOR RIBOSOME ACTIVITY AND SACCHAROMYCES-CEREVISIAE VIABILITY

Protein P0 in Saccharomyces cerevisiae is found only in the ribosomes and not free in a cytoplasmic pool like the structurally related acidi c P1 and P2 proteins. Analogously, P0 stays bound to the particles in conditions that release the other P proteins. Attempts to obtain yeast strains carrying an interrupted P0 gene by direct gene disruption tec hniques of different yeast strains always resulted in haploid cells ca rrying one disrupted and one intact copy of the gene. Disruption of th e unique P0 genomic copy seems to induce a duplication and occasionall y a chromosomal transposition of the gene. Conditional null mutants of P0 were then constructed carrying the P0 gene under the control of th e inducible GAL1 promoter. A 2-3-fold excess of P0 mRNA is found in th e conditional mutant when grown in galactose; however, only a small in crease of the P0 protein is detected in total cell extracts. No P0 pro tein is detected in the cell supernatant, indicating that, like the st andard ribosomal proteins and opposite to the other P proteins, the pr otein not bound to the ribosomes is degraded. Transfer of the mutants to the restrictive conditions causes, after some generations, a growth stop that finally leads to cell death. The growth decline is parallel ed by a reduction in the polysome number and the appearance of half me r particles as well as by an accumulation of 60 S particles deficient in P0 and in the acidic proteins P1 and P2. These results indicate tha t P0 is required for the interaction of the acidic P1 and P2 proteins with the ribosomes, and in its absence, deficient 60 S ribosomes are a ssembled which are inactive in protein synthesis resulting in cell let hality.