Abstract
The outcome of collisions between Bacillus subtilis phage Phi29 DNA polymerase and oppositely oriented transcription complexes has been studied in vitro. We found that the replication fork was unable to go past a transcription ternary complex stalled head-on. However, head-on collisions did not lead to a deadlock. Both DNA and RNA polymerase remained bound to the template and, when the halted transcription complex was allowed to move, the replication machinery resumed normal elongation. These results suggested that a replication fork that encounters an RNA polymerase head-on whose movement is not impeded would bypass the transcription machinery. Our results for head-on collisions between concurrently moving replication and transcription complexes are indeed consistent with the existence of a resolving mechanism. The ability of Phi29 DNA polymerase to resolve head-on collisions with itself during symmetrical replication of Phi29 DNA in vivo is likely to be related to its ability to pass a head-on oriented RNA polymerase.