Induced mutagenesis in Ustilago maydis. I. Isolation and characterization of a radiation-revertible allele of the structural gene for nitrate reductase.

A UV-revertible mutant of the nar1 structural gene for nitrate reductase was isolated in wild-type (nar+ nir+) Ustilago maydis. It proved to be vigorously revertible by gamma rays as well. Genetic analysis revealed that the strain carried a single, nonleaky, recessive allele (nar1-m) with an unusually high spontaneous reversion rate (approximately 3 X 10(-5)/div.). Reliable reversion frequencies were determined with a special agar medium that reduced the normally high level of residual growth observed on nitrate minimal agar. Radiation-induced reversion frequencies in the homozygous diploid were approximately twice those in the haploid. Following crosses to wild type, two revertants (one spontaneous and one UV-induced) were found to map at nar1. Although the molecular basis of nar1-m reversion is not known, available data suggest that some form of point mutation is involved.