HEAD-ON COLLISION BETWEEN A DNA-REPLICATION APPARATUS AND RNA-POLYMERASE TRANSCRIPTION COMPLEX

An in vitro system reconstituted from purified proteins has been used to examine what happens when the DNA replication apparatus of bacterio phage T4 collides with an Escherichia coli RNA polymerase ternary tran scription complex that is poised to move in the direction opposite to that of the moving replication fork. In the absence of a DNA helicase, the replication fork stalls for many minutes after its encounter with the RNA polymerase. However, when the T4 gene 41 DNA helicase is pres ent, the replication fork passes the RNA polymerase after a pause of a few seconds. This brief pause is longer than the pause observed for a codirectional collision between the same two polymerases, suggesting that there is an inherent disadvantage to having replication and trans cription directions oriented head to head. As for a codirectional coll ision, the RNA polymerase remains competent to resume faithful RNA cha in elongation after the DNA replication fork passes; most strikingly, the RNA polymerase has switched from its original template strand to u se the newly synthesized daughter DNA strand as the template.