XprG, a putative p53-like transcriptional activator, regulates production of extracellular proteases in response to nutrient limitation and may also have a role in programmed cell death. To identify genes that may be involved in the XprG regulatory pathway, 'xprG2' revertants were isolated and shown to carry mutations in genes which we have named 'sogA-C' (suppressors of 'xprG'). The translocation breakpoint in the 'sogA1' mutant was localized to a homolog of 'Saccharomyces cerevisiae VPS5' and mapping data indicated that 'sogB' was tightly linked to a 'VPS17' homolog. Complementation of the 'sogA1' and 'sogB1' mutations and identification of nonsense mutations in the 'sogA2' and 'sogB1' alleles confirmed the identification. Vps17p and Vps5p are part of a complex involved in sorting of vacuolar proteins in yeast and regulation of cell-surface receptors in mammals. Protease zymograms indicate that mutations in 'sogA-C' permit secretion of intracellular proteases, as in 'S. cerevisiae vps5' and 'vps17' mutants. In contrast to 'S. cerevisiae', the production of intracellular protease was much higher in the mutants. Analysis of serine protease gene expression suggests that an XprG-independent mechanism for regulation of extracellular protease gene expression in response to carbon starvation exists and is activated in the pseudorevertants.

In 'Aspergillus nidulans', production of extracellular proteases in response to carbon starvation and to a lesser extent nitrogen starvation is controlled by XprG, a putative transcriptional activator. In this study the role of genes involved in carbon catabolite repression and the role of protein as an inducer of extracellular protease gene expression were examined. The addition of exogenous protein to the growth medium did not increase extracellular protease activity whether or not additional carbon or nitrogen sources were present indicating that induction does not play a major role in the regulation of extracellular proteases. Northern blot analysis confirmed that protein is not an inducer of the major 'A. nidulans' protease, PrtA. Mutations in the 'creA', 'creB' and 'creC' genes increased extracellular protease levels in medium lacking a carbon source suggesting that they may have a role in the response to carbon starvation as well as carbon catabolite repression. Analysis of 'glkA4 frA2' and 'creAΔ4' mutants showed that the loss of glucose signalling or the DNA-binding protein which mediates carbon catabolite repression did not abolish glucose repression but did increase extracellular protease activity. This increase was XprG-dependent indicating that the effect of these genes may be through modulation of XprG activity.

Hexokinases catalyse the first step in glucose metabolism and play a role in glucose sensing in mammals, plants and fungi. We describe a new class of hexokinases that appear to be solely regulatory in function. The Aspergillus nidulans hxkD gene (formerly named xprF) encodes a hexokinase-like protein. We constructed hxkDΔ gene disruption mutants which showed increased levels of extracellular protease in response to carbon starvation. The hxkDΔ mutations are not completely recessive, indicating that the level of the gene product is critical. Transcript levels of hxkD increase during carbon starvation and this response is not dependent on functional HxkD. A gene encoding a second atypical hexokinase (HxkC) was identified. The hxkCΔ gene disruption mutant exhibits a phenotype similar, but not identical, to hxkDΔ mutants. As with hxkD, mutations in hxkC are suppressed by loss-of-function mutations in xprG, which encodes a putative transcriptional activator involved in the response to nutrient limitation. We show that GFP-tagged HxkD was found only in nuclei suggesting a regulatory role for HxkD. GFP-tagged HxkC was associated with mitochondria. Homologs of hxkC and hxkD are conserved in multi-cellular fungi. Genes encoding atypical hexokinases are present in many genome sequence databases. Thus, non-catalytic hexokinases may be widespread.