Structural alterations of the tRNA(m(1)G37) methyltransferase from Salmonella typhimurium affect tRNA substrate specificity

In Salmonella typhimurium, the tRNA(m(1)G37)methyltransferase (the product of the trmD gene) catalyzes the formation of m(1)G37, which is present adja cent and 3' of the anticodon (position 37) in seven tRNA species, two of wh ich are tRNA(CGG)(Pro) and tRNA(GGG)(Pro). These two tRNA species also exis t as +1 frameshift suppressor sufA6 and sufB2, respectively, both having an extra G in the anticodon loop next to and 3' of m(1)G37. The wild-type for m of the tRNA(m(1)G37)methyltransferase efficiently methylates these mutant tRNAs. We have characterized one class of mutant forms of the tRNA(m(1)G37 ) methyltransferase that does not methylate the sufA6 tRNA and thereby indu ce extensive frameshifting resulting in a nonviable cell. Accordingly, pseu dorevertants of strains containing such a mutated trmD allele in conjunctio n 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 m(1)G37 in the sufA6 tRNA. However, the sufB2 tRNA as well as the wild-type counterparts of these two tRNAs are efficient ly methylated by this class of structural altered tRNA(m(1)G37)methyltransf erase. Two other mutations (trmD3, trmD10) were found to reduce the methyla tion of all potential tRNA substrates and therefore primarily affect the ca talytic activity of the enzyme. We conclude that all mutations except two ( trmD3 and trmD10) do not primarily affect the catalytic activity, but rathe r 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 alter ed form of a tRNA. Moreover, we show that the TrmD peptide is present in ca talytic excess in the cell.