A dnaB analog specified by bacteriophage P1

Abstract

Bacteriophage P1 is shown to determine a product that can substitute in DNA replication for the protein specified by cistron dnaB of Escherichia coli. The viral dnaB analog ( ban) is repressed in the wild-type P1 prophage and expressed constitutively in plaque-forming mutants, P1 bac, described here. A particular P1 bac prophage allows lysogens of dnaBts bacteria to survive as colony-formers at temperatures that arrest DNA synthesis in the non-lysogens. The P1 bac prophage furthermore permits construction of an otherwise inviable strain bearing the unsuppressed amber mutation dnaB266. P1 bac prophages also suppress the groP character which is associated with certain dnaB mutations. The subclass of dnaB mutations called groP are those which prevent the growth of bacteriophage λ + at temperatures permissive for bacterial DNA synthesis, but allow the growth of certain λ mutants (λπ); π mutations have been mapped in gene P. Thus, λ + is enabled to grow in groP hosts by the presence of P1 bac-1 prophage. When dnaB protein is absent, however, as in the case of the unsuppressed amber mutant, the ban protein furnished by the P1 bac prophage does not support λ growth. Therefore, in the groP(P1 bac-1) lysogens both the dnaB and ban products are needed for λ growth, suggesting interactions between these E. coli and P1 proteins or their subunits. Mutations (termed ban) that prevent the expression of the dnaB analog determined by P1 have been obtained. P1 bac-1 ban-1, unlike P1 bac-1, fails to replicate in dnaBts hosts at temperatures non-permissive for bacterial DNA synthesis. Thus, the dnaB protein and its P1-determined analog can interchangeably fulfill an essential role in the replication of both the E. coli and P1 replicons. At permissive temperatures the lysogenization of certain dnaBts strains by P1 bac-1 ban-1 is very inefficient, probably as a result of negative complementation. Mutations bac-1 and ban-1 are closely linked on the P1 chromosome and their order relative to several amber mutations has been determined. Dominance studies of the alleles in transient diploids show that the ban-1 mutation is recessive to ban +. The bac-1 mutation, on the other hand, behaves in dominance tests as a DNA site mutation that permits constitutive expression in cis of the operon to which the ban gene belongs.