COMPETITION BETWEEN FRAMESHIFTING, TERMINATION AND SUPPRESSION AT THE FRAMESHIFT SITE IN THE ESCHERICHIA-COLI RELEASE FACTOR-II MESSENGER-RNA

Competition between frameshifting, termination, and suppression at the frameshifting site in the release factor-2 (RF-2) mRNA was determined in vitro using a coupled transcription-translation system by adding a UGA suppressor tRNA. The expression system was programmed with a plasmid containing a trpE-prfB fusion gene so that each of the products of the competing events could be measured. With increasing concentrations of suppressor tRNA the readthrough product increased at the expense of both the termination and the frameshifting product indicating all three processes are in direct competition. The readthrough at the internal UGA termination codon was greater than that at the natural UGA termination codon at the end of the coding sequence. The results suggest that this enhanced suppression may reflect slower decoding of the internal stop codon by the release factor giving suppression a competitive advantage. The internal UGAC stop signal at the frameshift site has been proposed to be a relatively poor signal, but in addition the release factor may be less able to recognise the signal with the mRNA in such a constrained state. Consequently, the frameshifting event itself will be more competitive with termination in vivo because of this longer pause as the release factor is decoding the stop signal.