Induced mutagenesis in Ustilago maydis. II. An in vivo biochemical assay.

UV gamma radiation-induced reversion to nar+ in a nar1-m nir1-1 strain of Ustilago maydis was found to occur under nongrowth conditions by performing the in vivo assay for functional nitrate reductase described by Resnick and Holliday (1971) who previously demonstrated that nonviable cells may still synthesize normal or near-normal levels of activity. Reversion frequencies of a signle gamma-irradiated culture were estimated in two cell populations by different methods: (A) among surviving clones after plating, and (B) among all cells (viable and nonviable) in suspension in the absence of postirradiation cell division. At gamma doses (300, 500 krad) corresponding to considerable cell killing (35%, 2% survival), reversion frequency by either method was the same. This supports the conclusion that mutation induction by gamma rays and its expression occur in nonviable cells with the same frequency as among survivors. If an error-prone repair system is assumed to be responsible for the observed gamma revertibility, then it is argued that this process is constituitive rather than inducible and that it is recombination-independent.