Fungi are capable of utilising numerous unrelated carbon and nitrogen sources, with the expression of the genes involved often strictly regulated. A major form of this regulation occurs as the phenomena of nitrogen metabolite repression (NMR) and carbon catabolite repression (CCR). Amide utilisation by A. nidulans has been extensively studied in our laboratory through analysis of the complex regulation of amdS. The primary substrate of amdS is acetamide, with expression affected by NMR, CCR and induction by acetate and omega-amino acids. The utilisation of formamide is mediated by a different structural gene, fmdS. We have cloned the fmdS gene and found that it encodes a protein belonging to the urease family of enzymes, rather than the amidase signature group. Unlike amdS, fmdS expression is primarily regulated via NMR and does not require addition of exogenous inducer. Despite the role of formamide as a nitrogen source only, fmdS displays a novel form of carbon regulation. Under carbon starvation conditions expression decreases dramatically, the opposite response to genes regulated by CCR. To further analyse fmdS regulation a series of reporter gene fusions were created, helping to determine specific sites of action for AreA (the major NMR regulatory protein). A binding site required for the A. nidulans CCAAT-binding factor has also been identified, with fmdS expression reduced twofold in a hapC deletion mutant. Sequence analysis and isolation of cDNAs show that a gene of unknown function lies directly 5' of fmdS, with 936bp of overlap with the fmdS transcript. The 5' gene has been disrupted through insertion of the riboB gene, with no phenotype yet discovered. The role of this gene may potentially be in nitrogen catabolism, as RT-PCR has shown it to be regulated in response to nitrogen limitation.
Fungal Genet. Newsl. 46 (Supl):
Full conference title:
Fungal Genetics Conference 20th
- Fungal Genetics Conference 20th (1999)