IDENTIFICATION OF THE ALTERED BASES IN MUTATED SINGLE-STRANDED DNA. I. IN VITRO MUTAGENESIS BY HYDROXYLAMINE, ETHYL METHANESULFONATE AND NITROUS ACID.

In vitro mutation of phage S13 (and ΦX174), which contain single-stranded DNA, was studied with hyclroxylamine, with ethyl methanesulfonate, and with nitrous acid. Both forward and reverse mutations could be studied selectively. It is argued that the reverse mutations studied are genuine reversions to the original genetic types. From the responses to the three mutagens, the following conclusions were obtained. (1) Hydroxylamine only mutates cytosine to thymine, and is the most specific of the three mutagens in its mutagenic action. (2) Ethyl methanesulfonate and nitrous acid mutate all four bases, but to varying degrees. The mutation of guanine to adenine by nitrous acid was established by extensive tests. (3) For each mutation studied, the altered base could be determined from the response to the three mutagens. (4) Mutation tests showed that only one of the two complementary DNA strands exists in the free phage population.