THE ASPERGILLUS-NIDULANS FLUG GENE IS REQUIRED FOR PRODUCTION OF AN EXTRACELLULAR DEVELOPMENTAL SIGNAL AND IS RELATED TO PROKARYOTIC GLUTAMINE-SYNTHETASE .1.

Mutations in the Aspergillus nidulans fluG gene disrupt the programmed induction of asexual sporulation and result in formation of fluffy co lonies that are characterized by undifferentiated cotton-like masses o f vegetative cells. We show that the fluG mutant phenotype is suppress ed when fluG mutant colonies are grown next to wild-type colonies even if the two strains are separated by dialysis membrane with a 6000- to 8000-dalton pore size. fluG encodes a cytoplasmically localized appro ximately 96,000-dalton polypeptide that is present at relatively const ant levels during vegetative growth and following developmental induct ion. Sequence analysis of fluG demonstrated that the carboxy-terminal 436 amino acids predicted by the 864-codon FluG open reading frame sha res approximately 28% identity with GSI-type prokaryotic glutamine syn thetases. We consider it unlikely that FluG functions in synthesis of glutamine but instead propose that FluG functions as a GSI-related enz yme in synthesizing an extracellular signal directing asexual sporulat ion and perhaps other aspects of colony growth. The relationships betw een fluG and other genes identified by fluffy mutants are discussed.