A UV-sensitive mutant of neurospora defective for photoreactivation

Abstract

A mutant ( uvs-1 upr-1) with enhanced UV sensitivity has been isolated from the uvs-1 upr-1 + mutant of N. crassa. The negative slope of the exponential UV dose-response curve of the uvs-1 upr-1 mutant is approximately 1.8 times that of the uvs-1 upr-1 + mutant. Although the mutant was selected for enhanced sensitivity in the “dark”, the mutant is simultaneously defective for photoreactivation (PR). The results of experiments in which PR was carried out at ice bath temperatures are consistent with the hypothesis that PR in microconidial strains of N. crassa, when carried out with a broad spectrum fluorescent light, is a composite of two types of PR: (1) direct (enzymatic) PR; (2) indirect (photochemical reversal of UV lesions). It is suggested that the uvs-1 upr-1 mutant is defective for direct (enzymatic) PR. Attempts to demonstrate in vitro repair of UV-irradiated b. subtilis transforming DNA by extracts from the uvs-1 upr-1 + mutant and the lack of such repair by extracts from the uvs-1 upr-1 double mutant, have been unsuccessful due to the presence of nucleases in the extracts. As judged by the ability of the enzyme extracts to solubilize tritium counts from labelled B. subtilis phage DNA, N. crassa produces a nuclease(s) capable of attacking unirradiated DNA as well as a nuclease(s) which degrades irradiated DNA with great efficiency. The uvs-1 upr-1 double mutant resulted from the alteration of a single nuclear gene ( upr-1) as revealed by linear tetrad analysis. Since it has not been possible to separate the upr-1 gene from its associated mating type gene (thus suggesting possible linkage between these loci), it has not been feasible to test the effects of the upr-1 gene on crossing over.