THE SPECTRUM OF ACRIDINE RESISTANT MUTANTS OF BACTERIOPHAGE-T4 REVEALS CRYPTIC EFFECTS OF THE TSL141 DNA-POLYMERASE ALLELE ON SPONTANEOUS MUTAGENESIS

Mutations in the ac gene of bacteriophage T4 confer resistance to acri dine-inhibition of phage development. Previous studies had localized t he ac gene region; we show that inactivation of T4 Open Reading Frame 52.2 confers the Ac phenotype. Thus, 52.2 is ac. The resistance mechan ism is unknown. The ac gene provides a convenient forward mutagenesis assay. Its compact size (156 bp) simplifies mutant sequencing and dive rse mutant types are found: base substitutions leading to missense or nonsense codons, in-frame deletions or duplications within the coding sequence, deletion or duplication frameshifts, insertions, complex mut ations, and large deletions extending into neighboring sequences. Comp arisons of spontaneous mutagenesis between phages bearing the wild-typ e or tsL141 alleles of DNA polymerase demonstrate that the impact of t he mutant polymerase is cryptic when total spontaneous mutant frequenc ies are compared, but the DNA sequences of the ac mutants reveal a sub stantial alteration of fidelity by the mutant polymerase. The patterns of base substitution mutagenesis suggest that some site-specific muta tion rate effects may reflect hotspots for mutagenesis arising by diff erent mechanisms. A new class of spontaneous duplication mutations, ha ving sequences inconsistent with misaligned pairing models, but consis tent with nick-processing errors, has been identified at a hotspot in ac.