Symmetric Replication of the Bacillus subtilis Chromosome

Synchronously replicating chromosomes in germinating spores of Bacillus subtilis were labeled near the origin with [(3)H]bromouracil. The label appeared in heavy-light DNA. When cell growth and chromosome replication were continued in unlabeled bromouracil, nearly all the tritium label was transferred to heavy-heavy DNA while the terminus remained unreplicated. This implies that both origins of the replicating chromosome can undergo reinitiation. Therefore, during multifork replication the chromosome takes on the symmetric "dichotomous" form rather than the asymmetric configuration predicted by the rolling circle model.