CHARACTERIZATION OF RCO-1 OF NEUROSPORA-CRASSA, A PLEIOTROPIC GENE AFFECTING GROWTH AND DEVELOPMENT THAT ENCODES A HOMOLOG OF TUP1 OF SACCHAROMYCES-CEREVISIAE

The filamentous fungus Neurospora crassa undergoes a well-defined deve lopmental program, conidiation, that culminates in the production of n umerous asexual spores, conidia, Several cloned genes, including con-1 0, are expressed during conidiation but not during mycelial growth, Us ing a previously described selection strategy, we isolated mutants tha t express con-IO during mycelial growth. Selection was based on expres sion of an integrated DNA fragment containing the con-10 promoter-regu latory region followed by the initial segment of the con-10 open readi ng frame fused in frame with the bacterial hygromycin B phosphotransfe rase structural gene (con10'-'hph). Resistance to hygromycin results f rom mutational alterations that allow mycelial expression of the con-1 0'-'hph gene fusion. A set of drug-resistant mutants were isolated; se veral of these had abnormal conidiation phenotypes and were trans-acti ng, i.e., they allowed mycelial expression of the endogenous con-10 ge ne, Four of these had alterations:at a single locus, designated ITO-I (regulation of conidiation), Strains with the rco-1 mutant alleles wer e aconidial, female sterile, had reduced growth rates, and formed hyph ae that coiled in a counterclockwise direction, opposite that of the w ild type, The four rco-1 mutants had distinct conidiation morphologies , suggesting that conidiation was blocked at different stages, Wild-ty pe rco-1 was cloned by a novel procedure employing heterokaryon-assist ed transformation and ligation-mediated PCR, The predicted RCO1 polype ptide is a homolog of Tup1 of Saccharomyces cerevisiae, a multidomain protein that mediates transcriptional repression of genes concerned wi th a variety of processes. Like tup1 mutants, null mutants of rco-1 ar e viable and pleiotropic. A promoter element was identified that could be responsible for RCO1-mediated vegetative repression of con-10 and other conidiation gene.,