Phosphorylation of the yeast equivalent of ribosomal protein S6 is not essential for growth

The yeast equivalent of ribosomal protein S6, known as S10, can be modified by the addition of two phosphates. The two adjacent serines that are likely to be subject to phosphorylation were deduced by comparison with the known sites of phosphorylation on rat liver S6. Using oligonucleotide mutagenesis, we altered the gene for S10 to replace these two serines with alanines. This mutant gene was introduced into a diploid yeast cell heterozygous for each of the two S10 genes. After sporulation, we obtained colonies in which the mutant gene was the only intact S10 gene. Although the ribosomes of these cells contained a full complement of S10, no phosphorylation of S10 was detected. These cells grow exponentially with a doubling time about 50% greater than that of control cells. We conclude that the phosphorylation of S10 is not essential for growth. However, the mutant gene in such cells is very unstable, frequently reverting to wild type, presumably by interaction with the disrupted host genes. We suggest that at some stage of the growth cycle there is strong selection for S10 that can be phosphorylated.