Figure 2. Models of replisome-RNAP collisions performed in vitro. (A) (left) The T4 replisome briefly pauses upon encountering a co-directional RNAP. The RNAP and transcript remain bound to the DNA as the replisome passes then an active RNAP elongation complex reforms using the same template. (right) The T4 replisome pauses after colliding with a head-on transcription complex then passes the RNAP which remains bound to the DNA. An active RNAP elongation complex reforms using the newly synthesized leading strand as a template. (B) (left) The E. coli replisome uses mRNA as a primer after colliding with a co-directional RNAP that is displaced from the DNA. (right) The E. coli replisome pauses after colliding with a head-on RNAP then resumes elongation after displacing the RNAP from DNA.