Biochemical genetics of Neurospora nuclease I: Isolation and characterization of nuclease (nuc) mutants

Characterization of Pi-repressible enzymes secreted in culture media by Neurospora crassa wild-type cells and null-type mutants

In wild-type mycelial cultures of Neurospora crassa under Pi-limited conditions, alkaline phosphatase, cyclic phosphodiesterases I, II, III, and IV, 5'-nucleotidase, acid and alkaline nucleases, RNase N1, and a newly detected endonuclease were secreted into the culture media. These enzymes were either not produced or were produced in very reduced levels in mutants nuc-1, -2, -3, -4, -5, -6, and -7 and cpd-4. The proteins were examined by polyacrylamide gel electrophoresis in a manner which allowed the identification of each of them.

Mutations affecting cyclic phosphodiesterases and adenylate cyclase in Neurospora

Two regulatory mutants in orthophosphate-regulated cyclic phosphodiesterase (cPDase), cpd-3 and cpd-4, were isolated and mapped proximal to arg-1 on L.G. IC and distal to arg-12 on L.G. IIR, respectively. cpd-3 showed short aerial hyphae with dense formation of conidia. The morphology was very similar to that of cr-1, cpd-3 and cr-1 had reduced levels of cyclic 3',5'-AMP, adenylate cyclase and cPDases (CPDase I, II and III in low phosphate condition) but had elevated levels of cyclic 3',5'-GMP. Although cr-1 showed an enhanced level and enhanced activation of heat activated cyclic phosphodiesterase (ha-PDE), this enzyme was not activated in cpd-3. cpd-4, nuc-1 and nuc-2 produced neither cPDase I, II, III, alkaline phosphatase nor ribonuclease N1 in low phosphate media. These mutants did not produce aerial hyphae or conidia when grown in low phosphate liquid medium. Activation of ha-PDE occurred in cpd-4, but not in nuc-1 and nuc-2.

Effect of phosphate levels on the synthesis of acid phosphatases (EC 3.1.3.2) in Neurospora crassa

When grown on high-phosphate medium, the wild-type strain 74A ofN. crassasynthesized two acid phosphatases, as shown by DEAE -cellulose chromatography. These purified enzymes showed heterogeneity on PAGE, low specific activities towards PNP-P, molecular weight values of at least 300000, no deviation from Michaelian behaviour, and great stability in 50 mM sodium acetate buffer, at pH 5·4, when kept at 54 °C. These acid phosphatases were synthesized in reduced amounts or not at all when the mould was grown under conditions of phosphate starvation, indicating that the level of phosphate also regulates the synthesis of the high molecular weight enzyme forms. When grown on high phosphate medium, thepho-3mutant strain also synthesized two acid phosphatases, whose purified enzymes showed no pronounced differences when compared to those synthesized by the wild-type strain in terms of electrophoretic analysis, specific activities towards PNP-P, molecular weight values, and Michaelian behaviour. However, one enzyme form had a higherKmvalue and a lower heat stability than the corresponding enzyme of the wild-type strain. Even though thepho-3locus might not be responsible for an alteration in the primary structure of the repressible acid phosphatase, it seems clear that the enzymes synthesized by the mould grown on low-or high-phosphate medium must share some structural features. Thus, the drastic differences observed in the molecular properties of the enzymes synthesized by the mould grown under conditions of phosphate starvation as opposed to phosphate repression might be due to an effect exerted by the level of inorganic phosphate in regulating the translation, post-translational modifications and/or excretion, but not necessarily the gene-directed synthesis of distinct mRNAs.

Effects of Neurospora nuclease halo (nuh) mutants on secretion of two phosphate-repressible alkaline deoxyribonucleases

Various recently isolated nuh mutants of Neurospora crassa (i.e., mutants which show reduced nuclease haloes on DNA-sorbose plates flooded with HCl) were mapped in several new genes or gene clusters and checked for effects on DNA repair and nuclease secretion. Some of them were found to be sensitive to MMS (methylmethane sulfonate) and sterile in meiosis. Release of nuclease activities into filtrates of liquid cultures was analyzed by DEAE-Sepharose chromatography. In the wild type, three alkaline deoxyribonuclease activities (A, B, and C) can be separated after growth in sorbose minimal media [Fraser, M. J. (1979). Nucleic Acids Res. 6: 231]. When strains were grown in phosphate-free DNA sucrose media, high (200-fold derepressed) DNase levels were found, and crude dialyzed filtrates could be chromatographed. Only two peaks were found, namely, those of DNase A, a Ca2+-dependent strand-nonspecific endonuclease, and DNase B, a ss-DNA-specific Mg2+-dependent exonuclease. Of the nuh mutants analyzed by one or both of these methods, many resembled the wild type. A few showed poor derepression, since their sorbose filtrates were normal, while profiles from DNA media lacked all peaks. These grew variably in liquid media with organic phosphates and probably produced suppressors, as was regularly found for nuc-2. Other mutants, which lacked specific peaks, gave the same results with both methods. One of these, nuh-7, produced no peaks at all but secreted unusually high amounts of protein.

Biochemical genetics of Neurospora nuclease II: Mutagen sensitivity and other characteristics of the nuclease mutants

Five newnucmutants ofNeurospora crassawere characterized for their relative sensitivities to different mutagens (UV, MNG, MMS), to mitomycin-C and to histidine; latter has been shown to inhibit the growth of certain UV sensitive mutants. These mutants were also compared for their capabilities for spontaneous mutation as determined by resistance to p-fluoro-phenylalanine. Based on these characterization, the mutants seem to belong to two groups. The first group includednuc-3andnuc-6which showed sensitivity to all mutagen tested and possessed capability for a very high frequency of spontaneous mutation (i.e. mutator effect). The second group includednuc-4, nuc-5andnuc-7; these were as resistant to different mutagens as the wild type strain, but possessed an antimutator effect (i.e. the frequency of spontaneous mutation by these three mutants were at least 0·5–100 × less than the wild type strains). There was some variation in these properties of mutants belonging to the two groups. Among all the fivenucmutants,nuc-3was characterized by extreme sensitivity to all mutagens. None of the fivenucmutants were sensitive to histidine. The properties ofnucmutants are discussed in relation to their possible role in DNA repair and recombination.