Imbalance of tRNAPro isoacceptors induces +1 frameshifting at near-cognate codons

Abstract

Increased expression of the CCU/CCA/CCG-decoding tRNAPro3 on a multicopy plasmid leads to suppression of several +1 frameshift mutations in Salmonella enterica serovar Typhimurium. Systematic analysis of the site of frameshifting indicates that excess tRNAPro3 promotes near-cognate decoding at CCC codons. Re-phasing of the reading frame can be achieved by a subsequent slippage of the tRNA onto a cognate codon in the +1 reading frame. Frameshifting appears to be due to an imbalance of CCC-cognate and near-cognate tRNAs, as the effect of excess tRNAPro3 on reading frame maintenance can be reversed by increasing simultaneously the concentration of the cognate tRNAPro2. Finally, the cmo5U modification present at position 34 of tRNAPro3, which allows this tRNA to decode CCU in addition to CCG and CCA, also affects frameshifting, indicating that the ability of the near-cognate tRNA to decode a cognate codon efficiently in the alternative reading frame is important for re-phasing of the reading frame.