A reduced level of charged tRNAArgmnm5UCU triggers the wild-type peptidyl-tRNA to frameshift

Frameshift mutations can be suppressed by a variety of differently acting external suppressors. The +1 frameshift mutation hisC3072 , which has an extra G in a run of Gs, is corrected by the external suppressor mutation sufF44 . We have shown that sufF44 and five additional allelic suppressor mutations are located in the gene argU coding for the minor tRNA Arg mnm 5 UCU and alter the secondary and/or tertiary structure of this tRNA. The C61U, G53A, and C32U mutations influence the stability, whereas the C56U, C61U, G53A, and G39A mutations decrease the arginylation of tRNA Arg mnm 5 UCU . The T-10C mutant has a base substitution in the -10 consensus sequence of the argU promoter that reduces threefold the synthesis of tRNA Arg mnm 5 UCU . The lower amount of tRNA Arg mnm 5 UCU or impaired arginylation, either independently or in conjunction, results in inefficient reading of the cognate AGA codon that, in turn, induces frameshifts. According to the sequence of the peptide produced from the suppressed -GGG-GAA-AGA- frameshift site, the frameshifting tRNA in the argU mutants is tRNA mnm 5 s 2 UUC Glu , which decodes the GAA codon located upstream of the AGA arginine codon, and not the mutated tRNA Arg mnm 5 UCU . We propose that an inefficient decoding of the AGA codon by a defective tRNA Arg mnm 5 UCU stalls the ribosome at the A-site codon allowing the wild-type form of peptidyl- tRNA mnm 5 s 2 UUC Glu to slip forward 1 nucleotide and thereby re-establish the ribosome in the 0-frame. Similar frame-shifting events could be the main cause of various phenotypes associated with environmental or genetically induced changes in the levels of aminoacylated tRNA.