Integration of bacteriophage Mu at host chromosomal replication forks during lytic development.

The target site for bacteriophage Mu integration in a lytic cycle of infection was investigated. DNA synthesis in five Hfr strains of Escherichia coli K-12 was synchronized by amino acid starvation and was allowed to proceed for 0, 8, or 15 min before infection. The Hfr cells were then infected with Mu and were subsequently mated with nonimmune F- recipient cells. Mating was interrupted mechanically at 5-min intervals and samples were assayed for infective centers. Conjugal transfer of Mu was delayed in Hfr strains that have transfer origins 15 map units or more from the E. coli replication origin, and the delays increased as the distance between an Hfr point of origin and the replication origin increased. When a gene A mutant of Mu was used for the infection, no infective centers were generated. Infection with a gene B mutant resulted in infective center formation only after long periods of mating. These data are most consistent with a model in which infecting Mu DNA or its progeny integrate at host chromosomal replication forks.