Current molecular models of genetic recombination invoke steps mediated by deoxyribonucleases. It is likely therefore that some mutants deficient in these enzymes would have defective recombination mechnisms. Mutants of Ustilago maydis deficient in extracellular and intracellular DNase activity have been isolated. Three of the extracellular mutants are recessive and non-complementing, as are four of the intracellular mutants. The loci for extracellular and intracellular DNase deficiency have been named nuc-1 and nuc-2 respectively, and they are linked.
In crosses where each parent was deficient in both extracellular and intracellular DNase activity, meiotic allelic recombination (gene conversion) within the nar locus was abolished, although crossing-over in one tested interval was apparently normal in frequency. In DNase-deficient diploids, no mitotic allelic recombination in the nar locus was observed and the distribution of crossing over was abnormal. Nuclease deficiency did not appear to have any strong effect on radiation sensitivity.