Mutual derepression in the P2–P4 bacteriophage system

Abstract

P4 lysogens are derepressed by infection with P2 as is evident from the induction of P4 phage production by P2 infection of P4 lysogens. Mutations interfering with P2 DNA synthesis greatly increase the P4 yield, but DNA replication by the infecting phage does not preclude P4 induction. Of four P2 mutations known to block spontaneous phage production by P2 lysogens, three ( cox-2, cox-3, cox-4) also prevent derepression of P4 lysogens by P2. These studies also revealed that, under certain conditions, P4 can complement P2 mutants deficient in gene B function which is required for P2 DNA replication. P2 lysogens are derepressed by infection with P4. This derepression leads to a P2 gene A-dependent P2 prophage replication and, in P2 lysogens mixedly infected with P2 and P4, also to the replication of the DNA of the coinfecting P2. If the derepressing P4 is deficient in gene a function, then derepression of the P2 lysogen will lead to the production of 10 or more P2 phages per cell, provided that P2 prophage excision is enhanced by the P2 nip mutation or is bypassed by infecting the P2 lysogen with P2 as well as with P4. Gene α proficient P4 interferes with the P2 phage production by P2 lysogens; possible causes for this interference are considered, including P4 DNA replication which is known to require gene a function.