Methionine Transfer Ribonucleic Acid from Folate-sufficient and Folate-deficient Streptococcus faecalis R

The formation of aminoacyl transfer ribonucleic acid derivatives containing a blocked α-amino group such as the formylmethionyl ester of methionine transfer ribonucleic acid (fMet-tRNAfmet) was studied in Streptococcus faecalis R. This bacterium cannot synthesize folate, and its growth is dependent upon the presence of either added folic acid or a mixture of products of biosynthetic reactions requiring the participation of tetrahydrofolate: serine, methionine, a purine, and thymine. Only extracts prepared from S. faecalis R cells grown on medium containing folic acid catalyzed the formation of an amino acid derivative esterified to tRNA which lacked a free α-amino group; of the amino acids tested, only methionine was a substrate for the formation of such an aminoacyl-tRNA derivative containing a blocked α-amino group. The methionyl-tRNA derivative was identified as N-formylmethionyl-tRNA. Extracts prepared from S. faecalis R cells grown on folate-free medium supplemented with serine, methionine, a purine, and thymine could not catalyze the formation of any detectable aminoacyl-tRNA species containing a modified α-amino group. The in vivo level of charged tRNAmet of exponentially grown S. faecalis R was estimated to be near 100% for both folate-sufficient and folate-deficient grown cells, and the total amount of tRNAfmet was the same under both culture conditions. However, methionyl-tRNAfmet was only formylated in cells grown in the presence of folic acid. Approximately 82 to 85% of the tRNAfmet of S. faecalis R in exponential growth stage in folate-containing medium was blocked to further formylation using 10-[14C]formyltetrahydrofolate as the formyl donor, whereas none of the tRNAfmet prepared from cells grown in the absence of folic acid was blocked to formylation. These results indicate that methionyl-tRNA in S. faecalis R grown in the absence of folic acid but in the presence of the supplements is neither formylated nor contains a blocked α-amino group and provide further evidence that formylation of methionyl-tRNAfmet is not required for the initiation of protein synthesis in S. faecalis R under certain defined conditions of culture.