The function of CreA, the carbon catabolite repressor of Aspergillus nidulans, is regulated at the transcriptional and post-transcriptional level

The creA gene of A. nidulans encodes a wide-domain regulatory protein media ting carbon catabolite repression. Northern blot analysis of creA mRNA reve aled a complex expression profile: the addition of monosaccharides to a car bon-starved culture of A. nidulans provoked a strong transient stimulation of creA transcript formation within a few minutes. In the case of repressin g carbon sources, creA mRNA levels were subsequently downregulated, whereas the high creA mRNA levels were maintained in a creA mutant strain and in t he presence of derepressing monosaccharides. A high creA transcript level i s essential to achieve carbon catabolite repression and is dependent on glu cose transport and, at least partially, on the creS gene product. Subsequen t downregulation of creA mRNA levels, on the other hand, is typical of carb on catabolite repression and requires a functional CreA recognition site in the creA promoter (and thus involves autoregulation) and formation of gluc ose-6-phosphate. Despite the presence of continuing high transcript levels of creA in the presence of derepressing carbohydrates, EMSA demonstrated th e presence of only low levels of a CreA-DNA complex in respective cell-free extracts. Upon transfer of carbon catabolite derepressed mycelia to catabo lite-repressing conditions, a CreA-DNA complex is formed, and this pro cess is dependent on de novo protein synthesis.