Chaos and Order in Spontaneous Mutation

Abstract

THIS story starts with one of those unanticipated turning points that mark the lives of most of us. Around 1960 I was working at the University of Illinois at Urbana using Newcastle disease virus (NDV), a tractable animal virus that infects chickens rather than humans. (Working previously with polioviruses, I had acquired extraordinarily high serum titers against all three strains.) The NDV plaque assay required that chicken embryos of the correct age be harvested and dissociated into single cells, which were then used to produce monolayers that became confluent in a few days. Virus samples were adsorbed to the monolayers, which were then incubated for a few more days while plaques formed. The regimen was cumbersome and contained day-long gaps that, as it turned out, were incompletely filled with the planning and analysis of experiments, the preparation and delivery of lectures, and the miscellaneous duties of a bottom-rung academic. As a graduate student I had followed the antics of the Caltech phage group and had also become aware of the mutagenic base analog 5-bromouracil. Over the next few years I was increasingly impressed by the early work on mutation by Ernst Freese and Seymour Benzer using phage T4. Ernst in particular was dissecting out mutational pathways with mutagens that seemed to be highly specific in their actions, for instance, the base analogs 5-bromouracil and 2-aminopurine that induced transitions in both directions and hydroxylamine that induced only G·C → A·T transitions. I often had occasion to UV irradiate NDV, and I wondered if the mutagenic specificity of UV irradiation could be ascertained by performing reversion tests on UV-induced mutations with base analogs, hydroxylamine, and proflavin, the specificity of the last having been demonstrated by Benzer and Sydney Brenner. Starting with a protocol developed by Raymond Latarjet for phage T4, I began to intercalate phage experiments into the free days provided by the NDV protocols. I quickly found that phages produced interesting results at least an order of magnitude more frequently than did NDV.