In Salmonella typhimurium, the
tRNA(m1G37)methyltransferase (the product of the
trmD gene) catalyzes the formation of m1G37,
which is present adjacent and 3′ of the anticodon
(position 37) in seven tRNA species, two of which are
tRNACGGPro and tRNAGGGPro.
These two tRNA species also exist as +1 frameshift suppressor
sufA6 and sufB2, respectively, both having an
extra G in the anticodon loop next to and 3′ of m1G37.
The wild-type form of the tRNA(m1G37)methyltransferase
efficiently methylates these mutant tRNAs. We have characterized
one class of mutant forms of the tRNA(m1G37)methyltransferase
that does not methylate the sufA6 tRNA and thereby
induce extensive frameshifting resulting in a nonviable
cell. Accordingly, pseudorevertants of strains containing
such a mutated trmD allele in conjunction with
the sufA6 allele had reduced frameshifting activity
caused by either a 9-nt duplication in the sufA6
tRNA or a deletion of its structural gene, or by an increased
level of m1G37 in the sufA6 tRNA. However,
the sufB2 tRNA as well as the wild-type counterparts
of these two tRNAs are efficiently methylated by this class
of structural altered tRNA(m1G37)methyltransferase.
Two other mutations (trmD3, trmD10) were
found to reduce the methylation of all potential tRNA substrates
and therefore primarily affect the catalytic activity of
the enzyme. We conclude that all mutations except two (trmD3
and trmD10) do not primarily affect the catalytic
activity, but rather the substrate specificity of the tRNA,
because, unlike the wild-type form of the enzyme, they
recognize and methylate the wild-type but not an altered
form of a tRNA. Moreover, we show that the TrmD peptide
is present in catalytic excess in the cell.