Effect of Purine and Pyrimidine Analogues on Reversion of a Tryptophan-requiring Strain of Escherichia coli

Abstract

WE suggested recently that nucleic acid precursors, altered in vivo by ultra-violet light, are involved as chemical intermediates in the induction of mutation¹. We have since reported that, in the absence of an adequate source of nitrogen, an immediate and rapid decline in the frequency of mutations induced by ultra-violet light occurs (‘mutation frequency decline’)². We demonstrated that the prevention of this decline depends on the availability immediately following exposure to radiation of a supply of amino-acids, and suggested that the amino-acids are involved in a process of temporary ‘stabilization’ of the ‘potential mutation’ (the modified purine or pyrimidine) in the cell prior to its effect in the induction of mutation (‘mutation stabilization’)². The potential mutation is ultimately ‘fixed’ into the cell and is no longer subject to those conditions which previously resulted in decline in mutation frequency (‘mutation fixation’)². Recently we have reported that the progression of ‘mutation fixation’ is closely correlated with the synthesis of ribonucleic acid in the culture, and suggested that the initial mutagenic event involves the incorporation of the modified nucleic acid precursor into ribonucleic acid³. We wish to report here experiments which determine the effect of certain purine and pyrimidine analogues on reversion induced by ultra-violet light in an auxotrophic strain of Escherichia coli.