Where does bacterial replication start? Rules for predicting the oriC region.

Three methods, based on DNA asymmetry, the distribution of DnaA boxes and dnaA gene location, were applied to identify the putative replication origins in 120 chromosomes. The chromosomes were classified according to the agreement of these methods and the applicability of these methods was evaluated. DNA asymmetry is the most universal method of putative oriC identification in bacterial chromosomes, but it should be applied together with other methods to achieve better prediction. The three methods identify the same region as a putative origin in all Bacilli and Clostridia, many Actinobacteria and gamma Proteobacteria. The organization of clusters of DnaA boxes was analysed in detail. For 76 chromosomes, a DNA fragment containing multiple DnaA boxes was identified as a putative origin region. Most bacterial chromosomes exhibit an overrepresentation of DnaA boxes; many of them contain at least two clusters of DnaA boxes in the vicinity of the oriC region. The additional clusters of DnaA boxes are probably involved in controlling replication initiation. Surprisingly, the characteristic features of the initiation of replication, i.e. a cluster of DnaA boxes, a dnaA gene and a switch in asymmetry, were not found in some of the analysed chromosomes, particularly those of obligatory intracellular parasites or endosymbionts. This is presumably connected with many mechanisms disturbing DNA asymmetry, translocation or disappearance of the dnaA gene and decay of the Escherichia coli perfect DnaA box pattern.