The interaction of induction, repression and starvation in the regulation of extracellular proteases in 'Aspergillus nidulans': evidence for a role for CreA in the response to carbon starvation

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.