Biosynthesis of ribosylthymine in the transfer RNA of Streptococcus faecalis: a folate-dependent methylation not involving S-adenosylmethionine

Ribosylthymine is not present in tRNA of Streptococcus faecalis grown in the absence of folic acid, although this methylated residue does occur in the tRNA of this organism when it is grown in the presence of folate. We have found that, unlike other methylated residues in RNA of S. faecalis and other organisms, the methyl moiety of ribosylthymine of the tRNA of folate-sufficient S. faecalis is not derived from S-adenosylmethionine. Preliminary evidence suggests that a folate derivative serves as the methyl donor in this methylation.