The initiation cascade for chromosome replication in wild-type and Dam methyltransferase deficient Escherichia coli cells.

'Newborn' Escherichia coli B/r cells, obtained by membrane elution, were used to study the cell cycles of wild-type and Dam methyltransferase mutants. In wild-type cells, initiation of chromosome replication was synchronous and tightly controlled. In dam mutants, initiation was altered, but not random. We propose that this is due to the absence of an initiation cascade caused by liberated DnaA molecules, and that this cascade normally synchronizes initiation. The dam- cells contained mainly two, three or four replication origins, and this affected nucleoid partitioning as well as cell division. In cultures growing with a 50 min doubling time, a variety of cell cycles were present and half the origins were used every 25 min. Some cells had a 25 min interdivision time, whereas others had an interdivision time longer than the generation time. Partitioning of nucleoids containing unequal numbers of replication origins could also be readily observed by fluorescence microscopy in the dam mutant. Based upon these observations we propose that the dam mutant is also an initiation cascade mutant.