Limiting DNA replication to once and only once

Abstract

In Escherichia coli cells, the origin of chromosomal replication is temporarily inactivated after initiation has occurred. Origin sequestration is the first line of defence against over-initiation, providing a time window during which the initiation potential can be reduced by: (i) titration of DnaA proteins to newly replicated chromosomal elements; (ii) regulation of the activity of the DnaA initiator protein; and (iii) sequestration of the dnaA gene promoter. This review represents the first attempt to consider together older and more recent data on such inactivation mechanisms in order to analyze their contributions to the overall tight replication control observed in vivo. All cells have developed mechanisms for origin inactivation, but those of other bacteria and eukaryotic cells are clearly distinct from those of E. coli. Possible differences and similarities are discussed.

Fig. 1. Schematic representation of mechanisms to limit initiation of chromosome replication in E. coli. The DnaA protein can bind ATP or ADP (A), bind to oriC and separate the DNA strands (B). After loading of the DnaB helicase, primase, and the elongation machinery the DnaA-bound ATP may be reduced to ADP by the β-clamp, which is part of the polymerase complex [yellow ellipse, (C)]. It should be noted that this ATP hydrolysis may continue as long as the β-clamp is loaded and not only shortly after initiation. The nascent DNA strands are unmethylated (red) and the SeqA protein binds hemimethylated DNA (D) and sequesters hemimethylated oriC. SeqA remains bound at the hemimethylated oriC long after the replication fork has passed datA, which titrates a large amount of DnaA (E). Note the difference in scale in the different panels.

Kirsten Skarstad

Anders Løbner-Olesen

Erik Boye