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Mol Microbiol. 2011 Jan;79(2):433-46. doi: 10.1111/j.1365-2958.2010.07453.x. Epub 2010 Nov 23.

The G157C mutation in the Escherichia coli sliding clamp specifically affects initiation of replication.

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1
Department of Cell Biology, Institute for Cancer Research, The Norwegian Radium Hospital, Oslo University Hospital, Oslo, Norway.

Abstract

Escherichia coli cells with a point mutation in the dnaN gene causing the amino acid change Gly157 to Cys, were found to underinitiate replication and grow with a reduced origin and DNA concentration. The mutant β clamp also caused excessive conversion of ATP-DnaA to ADP-DnaA. The DnaA protein was, however, not the element limiting initiation of replication. Overproduction of DnaA protein, which in wild-type cells leads to over-replication, had no effect in the dnaN(G157C) mutant. Origins already opened by DnaA seemed to remain open for a prolonged period, with a stage of initiation involving β clamp loading, presumably limiting the initiation process. The existence of opened origins led to a moderate SOS response. Lagging strand synthesis, which also requires loading of the β clamp, was apparently unaffected. The result indicates that some aspects of β clamp activity are specific to the origin. It is possible that the origin specific activities of β contribute to regulation of initiation frequency.

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