Conditional Switch between Frameshifting Regimes upon Translation of dnaX mRNA

Ribosome frameshifting during translation of bacterial dnaX can proceed via different routes, generating a variety of distinct polypeptides. Using kinetic experiments, we show that –1 frameshifting predominantly occurs during translocation of two tRNAs bound to the slippery sequence codons. This pathway depends on a stem-loop mRNA structure downstream of the slippery sequence and operates when aminoacyl-tRNAs are abundant. However, when aminoacyl-tRNAs are in short supply, the ribosome switches to an alternative frameshifting pathway that is independent of a stem-loop. Ribosome stalling at a vacant 0-frame A-site codon results in slippage of the P-site peptidyl-tRNA, allowing for –1-frame decoding. When the –1-frame aminoacyl-tRNA is lacking, the ribosomes switch into –2 frame. Quantitative mass spectrometry shows that the –2-frame product is synthesized in vivo. We suggest that switching between frameshifting routes may enrich gene expression at conditions of aminoacyl-tRNA limitation. [Display omitted] •–1 frameshifting predominantly occurs upon translocation of two slippery-site tRNAs•An alternative frameshifting pathway operates when aminoacyl-tRNA supply is limited•Hungry frameshifting is slow and independent of the mRNA secondary structure element•Switching between frameshifting routes can enrich coding capacity of the genome Caliskan et al. show that ribosomes can change the reading frame depending on aminoacyl-tRNA supply. Ribosome pausing at a hungry codon leads to –1 or –2 frameshifting independent of a regulatory mRNA element normally required for programmed frameshifting. Switching between frameshifting routes can enrich coding capacity upon aminoacyl-tRNA limitation.