Ultraviolet-sensitive mutants of Neurospora. I. Genetic basis and effect on recombination

Deoxyribonuclease-deficient mutants ofUstilago maydiswith altered recombination frequencies

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 ofUstilago maydisdeficient 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 namednuc-1andnuc-2respectively, and they are linked.

In crosses where each parent was deficient in both extracellular and intracellular DNase activity, meiotic allelic recombination (gene conversion) within thenarlocus was abolished, although crossing-over in one tested interval was apparently normal in frequency. In DNase-deficient diploids, no mitotic allelic recombination in thenarlocus was observed and the distribution of crossing over was abnormal. Nuclease deficiency did not appear to have any strong effect on radiation sensitivity.

Interaction of genes controlling ultraviolet sensitivity in Neurospora crassa

Two independently segregating ultraviolet (UV) sensitivity genes in Neurospora crassa interact synergistically resulting in UV sensitivity approximately twice that expected based on an evaluation of the sensitivities of the individual mutants. The mutant genes singly and together reduce photoreactivation (PR) in vivo although a PR enzyme is produced which exhibits normal activity in vitro.

Repair of ultraviolet light-induced damage to the deoxyribonucleic acid of Neurospora crassa

A method is described for labeling a specific pyrimidine in the deoxyribonucleic acid (DNA) of Neurospora crassa. In cells grown in the presence of [5-(3)H]-uridine, more than 97% of the radioactivity associated with the DNA had been incorporated into cytosine. The specific activity of the labeled DNA was approximately 3 x 10(3) counts per min per mug. The DNA was isolated by elution from hydroxyapatite columns with sodium phosphate buffer (0.40 m, pH 6.8). This procedure was used to demonstrate that in vegetative cells of N. crassa both photoreactivation and excision repair are operative, as measured by the removal of ultraviolet light-induced cytosine-containing dimers.

Simple and reliable method for replica plating Neurospora crassa

The details of a simple method for replica plating Neurospora crassa are described. The procedure has proved to be highly reliable for replica plating of colonies arising from either ascospores or conidia.