Biosynthesis of ribothymidine in the transfer RNA of Streptococcus faecalis and Bacillus subtilis. A methylation of RNA involving 5,10-methylenetetrahydrofolate.

The methyl moiety of ribothymidine in the tRNA of Streptococcus faecalis, Bacillus subtilis, and probably Bacillus cereus is dervied from a 1-carbon folate derivative and not S-adenosylmethionine. Micrococcus luteus (M. lysodeikticus) tRNA appears to be almost devoid of ribothymidine. S. faecalis tRNA lacking ribothymidine can be methylated in vitro with formation of ribothymidine. 5,10-Methylenetetrahydrofolate has been identified as the 1-carbon donor involved in ribothymidine formation in S. faecalis and implicated in the analogous reaction in B. subtilis. The reaction requires FADH2 and/or another reducing agent present in cell extracts for the reduction of the methylene moiety.