What is it about?
We investigated what happens when Escherichia coli cells are forced to copy their chromosome from a replication origin placed in an unusual position. In these cells, replication forks travel through some highly transcribed regions in the opposite direction from normal, creating head-on encounters with transcription machinery. We found that this arrangement slows growth and alters chromosome replication patterns, especially near highly active ribosomal RNA operons. These problems were partly relieved when the replication fork trap was removed or when RNA polymerase complexes were made easier for replication forks to displace. We also found that a previously reported fast-growing strain with this altered origin carried a large chromosome inversion that helped avoid many of the problematic head-on conflicts.
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Why is it important?
This study helps explain why bacterial chromosomes are organised so that DNA replication and transcription usually move in the same direction, particularly through heavily transcribed genes. Our results support the idea that avoiding head-on replication-transcription conflicts may be one factor that shaped the architecture of the E. coli chromosome. The work also suggests that the normal origin and termination arrangement helps keep chromosome copying balanced and less disruptive.
Perspectives
What stands out to us is that moving the replication origin gave a direct way to test how chromosome layout affects DNA replication in living E. coli cells. The combination of growth measurements, chromosome-wide replication profiles, and suppressor analysis allowed us to see both the problem and how cells could partly escape it. The large inversion in the earlier fast-growing strain was especially informative, because it showed how simply changing gene orientation could reduce the replication-transcription conflicts caused by the ectopic origin.
Dr. Christian J Rudolph
Brunel University
Read the Original
This page is a summary of: Shaping the landscape of the Escherichia coli chromosome: replication-transcription encounters in cells with an ectopic replication origin , Nucleic Acids Research, July 2015, Oxford University Press (OUP),
DOI: 10.1093/nar/gkv704.
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