Transfer RNA modifications that alter +1 frameshifting in general fail to affect −1 frameshifting

Using mutants ( tgt, mnmA ( asuE , trmU ) , mnmE ( trmE ) , miaA, miaB, miaE, truA ( hisT ) , truB ) of either Escherichia coli or Salmonella enterica serovar Typhimurium and the trm5 mutant of Saccharomyces cerevisiae , we have analyzed the influence by the modified nucleosides Q34, mnm 5 s 2 U34, ms 2 io 6 A37, Ψ39, Ψ55, m 1 G37, and yW37 on −1 frameshifts errors at various heptameric sequences, at which at least one codon is decoded by tRNAs having these modified nucleosides. The frequency of −1 frameshifting was the same in congenic strains only differing in the allelic state of the various tRNA modification genes. In fact, in one case (deficiency of mnm 5 s 2 U34), we observed a reduced ability of the undermodified tRNA to make a −1 frameshift error. These results are in sharp contrast to earlier observations that tRNA modification prevents +1 frameshifting suggesting that the mechanisms by which −1 and +1 frameshift errors occur are different. Possible mechanisms explaining these results are discussed.