Lessons from the genome sequence of Neurospora crassa: tracing the path from genomic blueprint to multicellular organism

We present an analysis of over 1,100 of the approximately 10,000 predicted proteins encoded by the genome sequence of the filamentous fungus Neurospora crassa. Seven major areas of Neurospora genomics and biology are covered. First, the basic features of the genome, including the automated assembly, gene calls, and global gene analyses are summarized. The second section covers components of the centromere and kinetochore complexes, chromatin assembly and modification, and transcription and translation initiation factors. The third area discusses genome defense mechanisms, including repeat induced point mutation, quelling and meiotic silencing, and DNA repair and recombination. In the fourth section, topics relevant to metabolism and transport include extracellular digestion; membrane transporters; aspects of carbon, sulfur, nitrogen, and lipid metabolism; the mitochondrion and energy metabolism; the proteasome; and protein glycosylation, secretion, and endocytosis. Environmental sensing is the focus of the fifth section with a treatment of two-component systems; GTP-binding proteins; mitogen-activated protein, p21-activated, and germinal center kinases; calcium signaling; protein phosphatases; photobiology; circadian rhythms; and heat shock and stress responses. The sixth area of analysis is growth and development; it encompasses cell wall synthesis, proteins important for hyphal polarity, cytoskeletal components, the cyclin/cyclin-dependent kinase machinery, macroconidiation, meiosis, and the sexual cycle. The seventh section covers topics relevant to animal and plant pathogenesis and human disease. The results demonstrate that a large proportion of Neurospora genes do not have homologues in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The group of unshared genes includes potential new targets for antifungals as well as loci implicated in human and plant physiology and disease.

Modification of Blue Light Photoresponses by Riboflavin Analogs in Neurospora crassa

The effect of riboflavin analogs on blue light responses in a riboflavin mutant of Neurospora crassa was studied. The analogs 1-deazariboflavin and roseoflavin, which have red-shifted absorption, acted as photoreceptors for the photosuppression and phase shifting of circadian conidiation by 540 nm light, but were ineffective as photoreceptors for the induction of carotenoid synthesis. These results provide addtional evidence implicating a flavin photoreceptor for at least two blue light responses of Neurospora.

Isolation and characterization of light-insensitive mutants of Neurospora crassa

As part of a genetic analysis of blue light photoreception in Neurospora, three mutants were isolated that do not exhibit photosuppression of circadian conidiation, i.e., they show periodic conidiation in constant light. The mutations have been given the designations lis-1, lis-2 and lis-3 ("light insensitive"). The three mutations segregate as single nuclear genes, are nonallelic and are recessive to wild type in heterokaryon tests. The linkage groups of the mutations are as follows: lis-1, I; lis-2, VI; and lis-3, V. The light -insensitive phenotype of the mutants is restricted to the photosuppression response; other responses such as photoinduced phase shifting of the conidiation rhythm and photoinduced carotenogenesis are not altered. The physiological or biochemical defects of the mutants have not been established, but they are not similar to previous reported cases (i.e., rib and poky) in which a reduction in light sensitivity has been observed.

Photoreception in Neurospora crassa: correlation of reduced light sensitivity with flavin deficiency

The effect of flavin deficiency on blue light responses in Neurospora crassa was studied through the use of two riboflavin mutants (rib-1 and rib-2). The photoresponses assayed were the suppression of circadian conidiation, the phase shifting of the circadian conidiation rhythm, and the induction of carotenoid synthesis. Flavin deficiency was induced in the rib-1 mutant by restrictive growth temperatures and in the rib-2 mutant by low levels of supplemental riboflavin. At 26 degrees C, a semirestrictive growth temperature, the rib-1 mutant is about 1/80th as sensitive to light for the photosuppression of circadian conidiation. Flavin deficiency in the rib-1 and rib-2 strains was effective in reducing the photosensitivity for phase shifting and carotenogenesis to about 1/16th and 1/4th of normal, respectively. Experiments with permissive temperatures, riboflavin supplementation, and revertants at the rib locus all indicated that the effects on light sensitivity were due solely to the presence of the rib mutations. These results provide evidence that one or more flavin photoreceptors are involved in the blue light responses of Neurospora.