Organization of sister origins and replisomes during multifork DNA replication in Escherichia coli

The replication period of Escherichia coli cells grown in rich medium lasts longer than one generation. Initiation thus occurs in the 'mother-' or 'grandmother generation'. Sister origins in such cells were found to be colocalized for an entire generation or more, whereas sister origins in slow-growing cells were colocalized for about 0.1-0.2 generations. The role of origin inactivation (sequestration) by the SeqA protein in origin colocalization was studied by comparing sequestration-deficient mutants with wild-type cells. Cells with mutant, non-sequesterable origins showed wild-type colocalization of sister origins. In contrast, cells unable to sequester new origins due to loss of SeqA, showed aberrant localization of origins indicating a lack of organization of new origins. In these cells, aberrant replisome organization was also found. These results suggest that correct organization of sister origins and sister replisomesis dependent on the binding of SeqA protein to newly formed DNA at the replication forks, but independent of origin sequestration. In agreement, in vitro experiments indicate that SeqA is capable of pairing newly replicated DNA molecules.