Fine-tuning of RecBCD expression by post-transcriptional regulation is required for optimal DNA repair in Escherichia coli

To preserve genome integrity, all living organisms have developed strategies to respond to chromosomal damage. One such response is the repair of DNA double-strand breaks (DSBs), one of the most toxic forms of DNA lesions. In E. coli, DSBs are repaired via the homologous recombination pathway, initiated by the RecBCD enzyme. RecBCD is essential for accurate chromosome maintenance but its over-expression can lead to reduced DNA repair ability. This apparent paradox suggests that RecBCD copy number may need to be tightly controlled within an optimal range. Using single-molecule fluorescence microscopy, we have established that RecB is present in very low abundance at mRNA and protein levels. RecB transcription shows high levels of fluctuations yet cell-to-cell protein variability remains remarkably low. We show that the post-transcriptional regulator Hfq binds to recBCD mRNAs and down-regulates RecB protein expression in vivo. Furthermore, when Hfq-mediated regulation is perturbed, we observe less effective noise reduction and reduced DNA repair capacity. Taken together, our results suggest a post-transcriptional regulatory mechanism where Hfq fine-tunes RecB expression by inhibiting RecB translation. This fine-tuning of RecB expression contributes to reducing noise in RecB protein expression and protects cells against the toxic consequences of too high RecBCD numbers.