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Review
, 55 (4), 978-85

Building a Bacterial Orisome: Emergence of New Regulatory Features for Replication Origin Unwinding

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Review

Building a Bacterial Orisome: Emergence of New Regulatory Features for Replication Origin Unwinding

Alan C Leonard et al. Mol Microbiol.

Abstract

Triggering new rounds of chromosomal DNA replication during the bacterial cell cycle is exquisitely regulated, ensuring both proper timing and one round per cycle stringency. A critical first step is stable unwinding of oriC, the chromosomal replication origin, by multiprotein orisome complexes comprising the AAA+ initiator DnaA and modulator proteins that bend DNA. Recently identified oriC-DnaA interactions in Escherichia coli raise important questions regarding the molecular mechanisms that regulate origin unwinding in bacteria. We describe staged binding of E. coli origin recognition proteins and suggest an unwinding switch based on interactions between DnaA-ATP and specialized oriC sites that must be filled during orisome assembly. By focusing multiple regulatory pathways on only a few key oriC DNA-protein interactions, this model includes an efficient way to control unwinding followed by orisome inactivation during the cell cycle. Future studies will determine whether this regulatory scheme is correct and whether it is generally applicable to other bacterial types.

Figures

Fig. 1
Fig. 1
Nucleotide sequence features of the E. coli replication origin, oriC. Wild-type oriC contains 245 base pairs. The sequences shown are all derived from the top DNA strand and are in their correct orientation on that strand. Position 3 of each DnaA recognition site (based on the orientation of R1) is shown in larger font. S–M denotes the DnaA recognition sites described by Speck and Messer (2001). The conserved 3′ recognition sequence shown for IHF is based on the study by Goodrich et al. (1990). Fis recognition site is based on compiled consensus recognition sequences described by Finkel and Johnson (1992). The entire nucleotide sequence of oriC can be viewed in the study by Cassler et al. (1995). See text for details.
Fig. 2
Fig. 2
Model for the assembly and disassembly of the E. coli orisome required to unwind oriC. DnaA is shown as red (ADP- or ATP-bound form retained from previous cell cycle) or blue (newly synthesized ATP-bound form) spheres. Modulators of DnaA binding: IHF, Fis and SeqA are shown in green. See text for details.

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