STRUCTURAL ALTERATIONS FAR FROM THE ANTICODON OF THE TRNA(GGG)(PRO) OF SALMONELLA-TYPHIMURIUM INDUCE -FRAMESHIFTING AT THE PEPTIDYL-SITE(1)

A total of 12 Salmonella typhimurium mutants were selected with mutati ons in the minor tRNA(GGG)(Pro) which suppress a +1 frameshift mutatio n in the hisD gene. This tRNA normally has 1-methylguanosine (m(1)G37) next to and 3' of the anticodon (position 37). Since the presence of m(1)G37 prevents frameshifting, some of the +1 frameshift suppressor d erivatives of tRNA(GGG)(Pro), had alterations in the primary sequence abolishing the formation of m(1)G37. However, several of the mutant tR NA(GGG)(Pro) species had a normal level of m(1)G37 and a normal-sized anticodon loop, showing that neither m(1)G37 deficiency, nor an oversi zed anticodon loop, is a prerequisite for +1 frameshifting. Moreover, base substitutions far from the anticodon, e.g. in the acceptor stem, DHU-loop and stem, and at the top of the anticodon stem, promoted +1 f rameshifting. When the frameshifting site (CCC-Uaa CCC is in the zero frame and a + 1 frameshift moves the ribosome to the CC-U codon) is ov erlapped by a nonsense codon (UAA), the efficiency of frameshifting de creased when release factor 1 was over-expressed and increased at an e levated temperature in a mutant with a temperature-sensitive release f actor 1. The frameshifting site (CCC-Uac) was also overlapped with the sense codon UAC, which is decoded by a tRNA species having a 2-methyl thio-cis ribozeatin (ms(2)io(6)A) at position 37. Mutations in the mia A gene affect the formation of this modified nucleoside and result in an A instead of ms(2)io(6)A37 in the tRNA. Such an undermodified tRNA is very inefficient in translation and the efficiency of frameshifting increased in a miaA1 mutant. These results suggest that the frameshif ting event occurs at the P-site, since the efficiency of frameshifting was sensitive to the decoding activity of the overlapping codon. We c onclude that tRNA with mutations far from the anticodon, with a normal -sized anticodon loop and having m(1)G37 induce +1 frameshifting at th e P-site. (C) 1997 Academic Press Limited.