The Aspergillus nidulans fluG gene is required for production of an extracellular developmental signal and is related to prokaryotic glutamine synthetase I.

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.