Two types of replication genes revealed through genetic studies. Temperature-sensitive mutants of E. coli were analyzed for the time needed for replication to stop after shifting cells to a nonpermissive temperature. DNA replication was observed by the uptake of [³H]thymidine into cellular DNA (measured as counts per minute, cpm) at a permissive temperature (30° C, open circles) and nonpermissive temperature (40° C, solid circles). (a) A gene giving a fast-stop phenotype encodes a protein involved in progression of the replication fork. (b) A gene giving a slow-stop phenotype encodes a protein involved in the initiation of replication.