Organization of a gene cluster expressed specifically in the asexual spores of A. nidulans.

We have investigated the organization and regulation of a 13.3 kb region of the Aspergillus nidulans genome that is preferentially expressed during conidiophore development. This cloned DNA segment codes for six polysomal poly(A)RNAs present in dormant asexual spores (conidia) at from 8 to 50 copies per cell. The genes encoding these RNAs occur once per haploid genome, are separate and distinct, appear to be colinear with their mature RNA products and are present in both polarities. Two of the genes have short regions of homology near their 5' ends, but otherwise the segment is internally unique. All of the RNAs are absent from or present at very low levels in wild-type somatic cells (Hyphae) and developing cultures of asporogenous mutant strains. In synchronously conidiating wild-type cultures, each of the RNAs can first be detected at a time coinciding with the appearance of mature spores. Thus this region comprises a cluster of tightly linked, discrete genes, which are all expressed at the same time, but not to the same extent, in a single differentiating cell type.