Tuesday, January 13, 2009

The replisome uses mRNA as a primer after colliding with RNA polymerase

Pomerantz RT, O'Donnell M.

The Rockefeller University, Howard Hughes Medical Institute, 1230 York Avenue, New York, New York 10021, USA.

Replication forks are impeded by DNA damage and protein-nucleic acid complexes such as transcribing RNA polymerase. For example, head-on collision of the replisome with RNA polymerase results in replication fork arrest. However, co-directional collision of the replisome with RNA polymerase has little or no effect on fork progression. Here we examine co-directional collisions between a replisome and RNA polymerase in vitro. We show that the Escherichia coli replisome uses the RNA transcript as a primer to continue leading-strand synthesis after the collision with RNA polymerase that is displaced from the DNA. This action results in a discontinuity in the leading strand, yet the replisome remains intact and bound to DNA during the entire process. These findings underscore the notable plasticity by which the replisome operates to circumvent obstacles in its path and may explain why the leading strand is synthesized discontinuously in vivo.

Another paper by Mike O'Donnell  highlighting the complexities of the DNA replication machinery and suggesting an exciting explanation to an 'ancient' observation regarding the discontinuous synthesis not only of the lagging strand (which is explained by the antiparallel disposition of the DNA strands that must be replicated by a single advancing holoenzyme), but also of the leading strand in vivo. Indeed, Okazaki, in his original work, observed this discontinuity in vivo and has been demonstrated by a number of in vivo studies since then.