Figure 2.

Collisions between backtracked RNA polymerase and RecBCD promote homologous recombination. Transcription by RNA polymerase (RNAP) and resection by RecBCD co-occur on the DNA template. RecBCD can initiate resection at a DNA break independent of ongoing transcription. A) GreA/B and DksA promote transcription elongation and prevent RNAP backtracking, allowing DNA degradation by RecBCD to continue. B) The SOS response in E. coli is activated by DNA breaks. ssDNA coated RecA filaments (RecA%) stimulates the autocatalytic cleavage of the transcriptional repressor LexA. Cleavage of LexA leads to the de-repression of genes in the SOS regulon, including RecBC and UvrD. C) A transcription error (yellow star) or UvrD along with ppGpp promote RNAP backtracking, increasing the chance for RecBCD to encounter backtracked RNAP. Stably bound backtracked RNAP stops RecBCD resection, which facilitates the switch to RecA loading and hence recombination.