Identification and characterization of mutations in Escherichia coli that selectively influence the growth of hybrid lambda bacteriophages carrying the immunity region of bacteriophage P22.

Mutations in two Escherichia coli genes, sipA and sipB, result in a specific inhibition of the growth of certain hybrid lambdoid bacteriophages, lambda immP22, that have the early regulatory regions and adjacent genes from bacteriophage P22. The sipB391 mutation maps near minute 56 and exerts the strongest inhibitory effect on the growth of the hybrid phages. The sipA1 mutation maps near minute 72 and plays an auxiliary role: enhancing the action of sipB391. Such a role is not limited to sipA1, since there is a similar enhancement by the nusA1 and nusE71 mutations. The Sip-imposed restriction on the growth of lambda immP22 phages is not observed if the phage carries a mutation in the c1 gene. Perhaps this reflects the fact that the c1 product regulates phage DNA replication and is a major determinant in the decision governing whether the phage takes the lytic or lysogenic pathway. Consistent with this idea is the observation that lambda immP22 DNA replication is severely inhibited in bacteria carrying the sipB391 mutation. It is suggested that sip mutations exaggerate the normal role of c1 in limiting lytic growth. This causes a failure in the expression of sufficient amounts of some or all of the lytic gene products required for phage growth.