Conidiation (asexual sporulation) in the filamentous fungus, Aspergillus nidulans, involves the formation of multicellular differentiated structures that produce pigmented, haploid conidia at precisely scheduled times. It has been shown that light is necessary for conidiation, but will only elicit development if irradiation occurs during a critical period of development. Furthermore, conidiation is induced by red light and suppressed by an immediate shift to far red light, a property that is characteristic of phytochrome mediated responses observed in plants. Several different classes of mutants that affect light dependent conidiation have been identified. Some mutants abolish light sensitivity, allowing conidiation to occur in the absence of light, while others affect the ability to conidiate at specific wavelengths. Thus, it appears that light signals are perceived in A. nidulans by a phytochrome like photoreceptor that transmits information by an unknown mechanism to genes that control asexual development. The objective of this proposal is to develop a detailed understanding of the mechanisms that regulate light mediated conidiation in this organism. The major aim of this work is to identify and characterize the red light photoreceptor through the molecular cloning of phytochrome related sequences. A phytochrome like clone has been isolated and experiments are proposed to compare the sequences and function of the A. nidulans phytochrome gene with known phytochrome genes from plants. These experiments include determining the DNA sequence of both genomic and cDNA clones, deducing the pattern of transcriptional expression, following the pattern of translational expression through the use of gene fusions and phytochrome specific antibodies, and examining the phenotype of gene disruptants. Another aim of this work is to identify interactions that occur within the photo transduction pathway. Mutants have been isolated in a cloned gene (acoD) that affect sensitivity to red light. Experiments are proposed to examine the transcriptional and translational activity of acoD in different growth and illumination conditions, to identify at the nucleotide level the mutational defects that cause red light insensitivity, and to identify gene interactions through the isolation and analysis of suppressor mutations. These studies will help elucidate the complex regulatory mechanisms that affect cellular differentiation is this eukaryote.
