Evidence against the folate-mediated formylation of formyl-accepting methionyl transfer ribonucleic acid in Streptococcus faecalis R.

The formylation of formyl-accepting methionyl transfer ribonucleic acid (tRNA fMet) was investigated in Sfreptococcus faecalis R, an organism incapable of synthesizing folic acid but whose growth does not require the vitamin under deiined conditions of culture. These studies were carried out because it was not clear how an organism with no folate could initiate protein biosynthesis. Growth of S. faecabk R on media free of folate but containing serine, methionine, thymine, adenine, and guanine was not affected by the presence of the folate antagonists, trimethoprim and aminopterin. However, growth of the organism on media lacking thymine and dependent upon the presence of either folic acid or 5formyltetrahydrofolate was severely inhibited by the analogues. It was demonstrated that extracts prepared from S. jaecalis R grown on a synthetic medium containing folic acid catalyzed the formylation of methionyltRNAfMet when either 10.formyltetrahydrofolate or formate was added as the source of the formyl group, whereas extracts prepared from cells of this organism grown on a medium free of folate but containing serine, methionine, thymine, adenine, and guanine catalyzed the formation of methionyltRNArMet only when lo-formyltetrahydrofolate but not when formate was used as the source of the formyl group. Extracts of these folate-free cells catalyzed the formylation of methionyl-tRNAf Met by formate if tetrahydrofolate was added to the assay mixture. These results indicate that S. faecdis R grown in the absence of folate but in the presence of the supplements does not contain tetrahydrofolate and therefore formylated methionyl-tRNAfMet may not be involved in initiation of protein synthesis in S. faecalis R, or the formylmethionyl ester of tRNAfMet (fMet-tRNArMet) may be formed by some as yet unknown enzyme system.