BASE ANALOG N-6-HYDROXYLAMINOPURINE MUTAGENESIS IN ESCHERICHIA-COLI -GENETIC-CONTROL AND MOLECULAR SPECIFICITY

We have studied the molecular specificity of the base analog N-6-hydro xylaminopurine (HAP) in the E. coli lad gene, as well as the effects o f mutations in DNA repair and replication genes on HAP mutagenesis. HA P induced base substitutions of the two transition types (A . T-->G . C and G . C-->A . T) at equal frequency. This bi-directional transitio n specificity is consistent with in vitro primer extension experiments with the Klenow fragment of DNA polymerase I in which we observed tha t either dTTP or dCTP were incorporated opposite HAP in an oligonucleo tide template. The spectrum of HAP-induced transitions was different f rom the spontaneous transitions in either a wild-type or a mismatch-re pair-defective (mutL) strain. Mutations in genes controlling excision repair, exonucleolytic proofreading, mismatch correction, error-prone (SOS) repair and 8-oxo-guanine repair did not affect HAP-induced mutag enesis substantially, However, an extensive deletion of several genes in the uvrB-bio region conferred supersensitivity to the lethal and mu tagenic effects of HAP, perhaps due to an effect on HAP metabolism. dn aE antimutator alleles reduced HAP-forward mutagenicity in allele-spec ific manner: dnaE911 reduced it several fold, while dnaE915 abolished it almost completely. The results obtained are consistent with the ide a that HAP is mutagenic in E. coli via a pathway generating replicatio n errors.