Photoreceptor Apparatus of the Fungus Neurospora crassa

Light governs a number of vitally important functions of the ascomycete fungus Neurospora crassa by controlling the expression of genes responsible for differentiation of reproductive structures, synthesis of secondary metabolites, and the circadian oscillator activity. Illumination also affects electrogenic processes in cell membranes and the activity and molecular organization of some enzymes. A major but probably not the sole photoreceptor pigment in Neurospora cells is WCC, a heterodimeric complex formed by the PAS-domain-containing polypeptides WC-1 and WC-2, the products of genes white collar-1 and white collar-2. Mutations in any of these genes arrest the majority of fungus responses to light. The photoreceptor belongs to the recently discovered large group of nonhomologous light-sensitive proteins whose molecules bind flavin coenzymes as photosensor chromophores. The photosignal transduction is started by excitation and photochemical activity of an excited FAD molecule noncovalently bound by the LOV domain (a specialized variant of the PAS domain of the WC-1 apoprotein). The presence of zinc fingers (the motives recognizing GATA sequences in promoter) in both WC-1 and WC-2 proteins suggests that these motifs might function as transcription factors. However, a critical analysis of the photoinduction mechanism has shown that the promoters of light-sensitive genes do not contain a common cis-acting element, thereby suggesting that some alternative mechanisms underlie photoregulated gene activity.[PUBLICATION ABSTRACT]