Control of chromosome replication in thymine-requiring strains of Bacillus subtilis 168.

Study of the replication pattern of a number of B. subtilis 168 strains under controlled physiological conditions revealed great interstrain variation in control of replication. Replication patterns were calculated from ratios of purA16/leu-8 and purA16/metB5 transformation frequency. The thymine-independent strains are under strict regulation with an average of one replication position per chromosome during log phase. After starvation for required amino acids or sporulation, the chromosome is in a completed state with no replication forks (class I). In contrast, several thymine-requiring strains (class III) have an average of three to four replication positions per chromosome during log phase (multiforked replication) of which one to two remain uncompleted after amino acid starvation or sporulation. The other thymine-requiring strains studied are intermediate (class II) in that they have an average of two replication positions per chromosome during log phase and one after amino acid starvation or sporulation. Pulse chase experiments indicate that the deoxyribonucleic acid which is close to the chromosomal origin on each branch of the multiforked chromosome is bound to a rapidly sedimenting cellular fraction, presumably membrane.