Translational frameshifting in the Escherichia coli dnaX gene in vitro

Production of the gamma subunit of Escherichia coli DNA polymerase III holoenzyme is dependent on a very efficient translational frameshif in the dnaX gene. I used an E. coli in vitro translation system to analyze the mechanism of this frameshifting event. In this system, gamma was produced almost to the same extent as the inframe translation product, tau, suggesting that efficient frameshifting was reproduced in vitro. Coupling with transcription was not necessary for frameshifting. Addition of purified tau or gamma had no effect on the frameshifting process suggesting the absence of direct feedback regulation. By use of mutant genes, a strong pausing site was identified at or very close to the frameshift site. This pausing was apparently caused by a potential stem-loop structure which was previously shown to enhance frameshifting. Thus, enhancement of frameshifting by this putative stem-loop seems to be mediated by the translation pausing at the frameshift site. Despite the apparent structural similarity of the dnaX frameshift site to that of the eukaryotic retroviral genes, dnaX mRNA synthesized in vitro failed to direct the production of gamma in eukaryotic translation systems. This suggests that frameshifting in the dnaX gene depends on components specific to the E. coli translation system.