Mutations affecting cyclic phosphodiesterases and adenylate cyclase in Neurospora

Identification ofvib-1, a Locus Involved in Vegetative Incompatibility Mediated byhet-cinNeurospora crassa

A non-self-recognition system called vegetative incompatibility is ubiquitous in filamentous fungi and is genetically regulated by het loci. Different fungal individuals are unable to form viable heterokaryons if they differ in allelic specificity at a het locus. To identify components of vegetative incompatibility mediated by allelic differences at the het-c locus of Neurospora crassa, we isolated mutants that suppressed phenotypic aspects of het-c vegetative incompatibility. Three deletion mutants were identified; the deletions overlapped each other in an ORF named vib-1 (vegetative incompatibility blocked). Mutations in vib-1 fully relieved growth inhibition and repression of conidiation conferred by het-c vegetative incompatibility and significantly reduced hyphal compartmentation and death rates. The vib-1 mutants displayed a profuse conidiation pattern, suggesting that VIB-1 is a regulator of conidiation. VIB-1 shares a region of similarity to PHOG, a possible phosphate nonrepressible acid phosphatase in Aspergillus nidulans. Native gel analysis of wild-type strains and vib-1 mutants indicated that vib-1 is not the structural gene for nonrepressible acid phosphatase, but rather may regulate nonrepressible acid phosphatase activity.

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.

Regulation of synthesis and secretion of acid and alkaline phosphatases in Neurospora crassa

We show that N. crassa represses the production of acid phosphatase at pH higher than 8.0, irrespective of the carbon source used, whereas production was stimulated by sucrose at slightly acidic pH. The same profile of acid phosphatase production was observed in the pho-2A, pho-3A, nuc-1A, nuc-2A and pregc mutant strains. We also show that acid phosphatase synthesized by the pregc mutant strain grown on high phosphate medium has pronounced differences when compared to the enzyme synthesized by the wild-type strain grown on low phosphate medium in terms of heat stability, steady-state kinetic properties and DEAE-cellulose chromatography. In addition, the synthesis and/or secretion of only phosphate-repressible alkaline phosphatase is affected by mutations in acu-1, and acu-5 and acu-7 genes. These results, which indicate distinct pathways for the synthesis and secretion of acid and alkaline phosphatases in N. crassa, contradict the dosage titration model proposed by Metzenberg et al. (1974) whereby the synthesis of these enzymes should occur through a single hierarchical regulatory circuit as a response to phosphate starvation.