Aspergillus nidulans

Light-dependent conidiation in the filamentous ascomycete, Aspergillus nidulans, is contingent on the allelic state of the velvet (veA) gene. Light dependence is abolished by a mutation in this gene (veAl) , which allows conidiation to occur in the absence of light. We have isolated and characterized six extragenic suppressors of veAl that restore the light-dependent conidiation phenotype. Alleles of four genes, defined by complementation tests, were subjected to extensive genetic and phenotypic analysis. The results of light-dark shifting experiments and the phenotypes of double mutant combinations are consistent with the possibility that the expression of the light-dependent phenotype is regulated by specific interactions of the suppressor gene products with the velvet gene product and with each other. C ONIDIATION (asexual sporulation) in the filamentous ascomycete, Aspergillus nidulans, is a complex process involving the induction and coordinate regulation of many genes. The expression of these genes leads to the formation of multicellular differentiated structures called conidiophores which, in turn, produce pigmented, haploid conidia at precisely scheduled times (AXELROD 1972; CLUTTERBUCK 1969; MARTINELLI and CLUTTERBUCK 197 1 ; TIMBERLAKE 1980). In wild-type strains, conidiation is induced by exposure of mycelia to red light, but is suppressed by an immediate shift to far red light, reminiscent of the phytochrome-mediated responses of higher plants (MOONEY and YAGER 1990). However, light-mediated conidiation is contingent on the allelic state of the velvet (veA) gene. Specifically, strains that are wild-type (veA+) at this locus display the lightdependent conidiation phenotype, while strains bearing a mutation at this locus, designated v e A l , conidiate regardless of the presence or absence of light. The v e A l mutation shows a wide range of pleiotropic effects. veAI strains fail to produce the profuse aerial hyphae normally present in wild-type colonies, showing vigorous conidiation instead (KAFER 1965). CHAMPE et al. (1981) demonstrated that v e A l strains are acleistothecial at high temperature (42"), and noted that a 36,000 M.W. protein, which is absent in the v e A l mutant, accumulates in veA+ strains. Although the nature and function of the velvet gene are unknown, we have suggested that the veA gene product may function as a negative regulator or may affect ' Present address: Department of Genetics, University of Georgia, Athens, ' Present address: Department of Immunology and Medical Microbiology, Georgia 30602. University of Florida College of Medicine, Gainesville, Florida 32610. Cknetics 1 2 6 869-874 (December, 1990) the activity of a negative regulator, which controls the expression of certain conidiation-specific genes (MooNEY and YAGER 1990). Analysis of extragenic suppressors is a powerful method in dissecting complex gene interactions (e.g., JARVIK and BOTSTEIN 1975). The underlying hypothesis of this strategy is that if two gene products interact, a deleterious mutation in one gene product can be suppressed by a compensating mutation in the interacting gene product. Accordingly, to help elucidate the function of the velvet gene and to identify other genes involved in the light response, we have isolated and characterized six extragenic suppressors of the v e A l mutation that restore the light-dependent conidiation phenotype. MATERIALS AND METHODS Aspergillus strains and genetic techniques: Strains used in this study are listed in Table 1. Genotypes are designated as in CLUTTERBUCK (1984). Standard genetic techniques were employed throughout this study (PONTECORVO et al. 1953; CLUTTERBUCK 1977; KAFER 1977). Mitotic haploidization of heterozygous diploids was performed as described in MCCULLY and FORBES (1965), substituting benlate as the haploidizing agent (HASTIE 1970). Media and growth conditions: The complete growth medium and general culture techniques described by YAGER, KURTZ and CHAMPE (1 982) and BUTNICK et al. (1984) were used throughout this study. Surface grown colonies requiring illumination were maintained in i cubators equipped with GE 20 W Broad Spectrum fluorescent light bulbs positioned 20 cm from the agar surface (average illumination between 10 and 13 W m-'). Dark conditions were obtained by loosely wrapping single plates in aluminum foil. All incubations were performed at 32". Submerged growth rates were determined as described in MOONEY and YAGER (1 990). The growth of fungi in submerged culture may obey either a cube-root or exponential relationship 870 J. L. Mooney, D. E. Hassett and L. N. Yager