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 colonies that are characterized by undifferentiated cotton-like masses of vegetative cells. We show that the fluG mutant phenotype is suppressed 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 approximately 96,000-dalton polypeptide that is present at relatively constant levels during vegetative growth and following developmental induction. Sequence analysis of fluG demonstrated that the carboxy-terminal 436 amino acids predicted by the 864-codon FluG open reading frame shares approximately 28% identity with GSI-type prokaryotic glutamine synthetases. We consider it unlikely that FluG functions in synthesis of glutamine but instead propose that FluG functions as a GSI-related enzyme in synthesizing an extracellular signal directing asexual sporulation and perhaps other aspects of colony growth. The relationships between fluG and other genes identified by fluffy mutants are discussed.