Chromosome replication pattern in dam mutants of Escherichia coli

The replication cycle of Escherichia coli dam mutants was analysed and compared with that of isogenic Dam+ strains. Marker frequency analyses indicated no gross difference between the strains. In the Dam- as well as in the Dam+ bacteria, initiation most likely occurs at oriC, replication forks move at a constant and invariant velocity, and termination takes place in the terC region. An analysis of replication terminator activity indicated that this activity is unaffected by the methylation status. Taken together with previous results, our data are compatible with Dam methylation controlling initiation timing but no subsequent step of the replication process.