Origin and Sequence of Chromosome Replication in Escherichia coli B/r

Abstract

The initial rates of induced synthesis of trypto-phanase, [beta]-galactosidase and D-serine deaminase were measured in relation to the chromosome replication cycle of E. coli B/r. Exponentially growing cultures were exposed briefly to 14C-thymidine or the appropriate inducers (or both) and the amount of label or enzyme (or both) in cells of different ages found by measuring these quantities in their progeny. The rates of induced synthesis of the 3 enzymes increased abruptly at about 4, 20 and 34 min., respectively, after the start of a round of replication lasting 40 min. By matching this sequence to the ind, lac and Dsd loci on the genetic map of E. coli K-12, it was estimated that replication began at about 8 o''clock (60 min.) and proceeded clockwise. In rapidly growing cells, the sequence during the division cycle was consistent with the concept that rounds of replication overlapped.