According to the prevailing model, frameshift-suppressing tRNAs with an extra nucleotide in the anticodon loop suppress +1 frameshift mutations by recognizing a four-base codon and promoting quadruplet translocation. We present three sets of experiments that suggest a general alternative to this model. First, base modification should actually block such a four-base interaction by two classical frameshift suppressors. Second, for one Salmonella suppressor tRNA, it is not mutant tRNA but a structurally normal near cognate that causes the +1 shift in-frame. Finally, frameshifting occurs in competition with normal decoding of the next in-frame codon, consistent with an event that occurs in the ribosomal P site after the translocation step. These results suggest an alternative model involving peptidyl-tRNA slippage at the classical CCC-N and GGG-N frameshift suppression sites.

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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 adjacent and 39 of the anticodon (position 37) in seven tRNA species, two of which are tRNA CGG Pro and tRNA GGG Pro . 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 39 of m 1 G37. The wild-type form 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 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 m 1 G37 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(m 1 G37)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.

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