Functional analysis of DNA sequences required for conidium-specific expression of the SpoC1-C1C gene of Aspergillus nidulans.

The SpoC1-C1C gene is centrally located within the A. nidulans conidium-specific SpoC1 gene cluster. With one exception, the 14 genes within the cluster are coordinately regulated. C1C transcript is first detected late in conidiation, coincidental with the appearance of mature conidia, and accumulates approximately 1000-fold in conidia. We show that C1C expression is restricted to conidia, with mRNA abundance decreasing immediately after induction of germination. C1C transcription and translation are not temporally separated and, similar to C1C RNA abundance, a C1C::beta-galactosidase fusion protein is first detected with the appearance of mature conidia and decreases after induction of germination. Cell-specific C1C expression requires both a position-dependent mechanism of regulation, responsible for repression in hyphae, and a position-independent mechanism of regulation, responsible for developmental expression. We show by functional analysis of upstream DNA sequences that a 10-bp sequence and two adjacent 6-bp direct repeats are necessary for position-independent, condium-specific expression of both the intact C1C gene and the reporter gene. At least one repeat (CAACAT) is required for normal levels of expression. We find that the C1C gene is not a direct target of the BrlAp and AbaAp developmental regulators, but of a yet unidentified conidium-specific transcriptional activator.