Book of Abstracts: Albany 2011
June 14-18 2011
©Adenine Press (2010)
A Topological View of Transcription-Replication Collisions
Positive supercoiling builds-up ahead of progressing forks and DNA topoisomerases counteract this effect allowing a smooth advance (1). But the processivity of helicases outcomes topoisomerases and as completion of replication approaches the DNA ends up with an excess of positive supercoiling. This supercoiling in the unreplicated portion can migrate to the replicated portion by swivelling the forks. Here it gives rise to right-handed catenanes once replication is over (2). In bacteria, topoisomerase IV (Topo IV) is responsible for the decatenation of sister duplexes (3), but Topo IV is more efficient in the elimination of left-handed crosses (4). This apparent contradiction is known as the Topo IV paradox (5) and recent data suggests that supercoiling of the newly made sister duplexes plays an essential role to solve it (6,7). The scenario changes significantly if transcription and replication forks progress in opposite directions. The accumulation of positive supercoiling in between would slow down the advancing forks and this might allow topoisomerases to cope helicases in order to maintain the replicon negatively supercoiled up to the end. If negative supercoiling ahead of the forks migrates to the replicated portion, it would give rise to left-handed catenanes once replication is over. Among other consequences, it is known this enhances the formation of intermolecular knots during replication (8). Here we combined classical genetics with high-resolution two-dimensional agarose gel electrophoresis and atomic force microscopy to investigate this problem in bacterial plasmids and yeast minichromosomes as prokaryotic and eukaryotic model systems, respectively.
This research was supported by grants BFU2008-00408/BMC to JBS and BFU2007-62670 to PH from the Spanish Ministerio de Ciencia e Innovación.
Department of Cell Proliferation & Development