Mutator phenotype of induced cryptic coliphage lambda prophage

Abstract

These studies are based on the isolation of λ replication defective mutants that had acquired multiple point mutations within λ replication initiation genes O and P in a cryptic prophage (Hayes et al., 1998). Each mutant cell arose after shifting wild type cells with a cI[Ts] cryptic λ prophage deleted for int-kil, and from ren into E. coli, from 30°C to 42°C. Derepression of the trapped cryptic prophage kills the host cells (designated as RK⁺). Rare colony forming units survive and were designated as RK⁻ mutants. This led to a hypothesis that λ replication-triggered cell stress provokes mutator activity, i.e., increases the frequency of replication errors within the simultaneously replicating chromosome of the host E. coli cells. We tested this hypothesis by asking three questions: (1) Do unselected, untargeted (with no link to λ fragment) auxotrophic mutations appear within the RK⁻ mutant population selected from RK⁺ culture cells? (2) Is replication initiation from the cryptic λ fragment, or, alternatively, just expression of one or more λ genes required for the appearance of the unselected auxotrophic mutations? (3) Do E. coli functions participate in the appearance of unselected auxotrophic mutations within the RK⁻ mutant population? Our results indicate that auxotrophic mutations unlinked to the λ fragment appeared at high frequency within RK⁻ mutants. RK⁻ auxotrophs arising on rich medium were identified by screening the survivor clones for growth on minimal medium. The appearance of RK⁻ auxotrophic colonies at high frequency (>1 per 100 RK⁻ mutants) leads us to conclude that auxotrophic mutations arise during the independent selection for RK⁻ mutants. Conditions that inhibited λ fragment induction fully suppressed the mutator phenotype. Mutation of host dnaB such that the helicase does not support replication initiation from the induced λ fragment completely suppressed host cell killing, but not the appearance of auxotrophic mutations. We asked if E. coli error-prone polymerases IV and V, or gene functions regulated as part of the host SOS response contributed to the provoked mutator phenotype and observed no close correlation. We demonstrated that the RK⁺ starting cells did not have a distinct intrinsic mutator activity in several ways, including moving the cryptic λ fragment to different E. coli host cells, blocking λ fragment induction by the addition of a cI⁺ plasmid to eliminate λ gene expression at high temperatures, and independent assays for spontaneous rifampicin resistance. We found that the induced mutator phenotype associated with the appearance of untargeted auxotrophs was linked to the expression of lambda gene P, and did not require replication initiation from the cryptic λ prophage. We also found that the mutator phenotype of the induced cryptic λ fragment increased the frequency of rifampicin resistant colonies among the RK⁻ mutant population.