FIG 1.

Biosynthesis of ho⁵U derivatives in bacterial tRNA. In E. coli and other gammaproteobacteria, CmoA and CmoB convert AdoMet to carboxy-AdoMet and transfer the carboxymethyl group to ho⁵U to give cmo⁵U. The latter is then methylated by CmoM to give the terminal modification mcmo⁵U. In B. subtilis and most other bacteria, ho⁵U is methylated directly by the recently identified TrmR (YrrM) methyltransferase (22). The genes required for ho⁵U synthesis have yet to be described and are the subject of this work.