In Aspergillus nidulans, a DNA-binding complex, PENR1, was shown to bind to two CCAAT-boxcontaining DNA elements located in the promoter regions of the bidirectionally oriented penicillin biosynthesis genes acvA and ipnA, and of the aat promoter. Here, partial purification of PENR1 and western blotting using anti-HAPC sera indicated that the previously identified HAPC protein, which was suggested to be part of the CCAAT-binding complex AnCF, is also part of PENR1. This was confirmed by band shift assays using protein extracts of a ∆hapC strain which exhibited no PENR1 DNA-binding activity. Supershift assays and immunoprecipitation analysis using anti-HAPC sera provided evidence that HAPC is part of the PENR1 complex. In ∆hapC strains, penicillin production was reduced, as was expression of both an ipnA-lacZ and aat-lacZ gene fusion. Hence, HAPC-containing PENR1 appears to act as an activator on ipnA and aat expression. However, deletion of hapC had little effect on acvA expression during a fermentation run in fermentation medium. Previous results which had shown that specific deletion of the PENR1-binding site between acvA and ipnA resulted in a strong increase of expression of an acvA-uidA gene fusion, together with the present data, suggest the possibility of the existence of a repressor protein that binds close to or overlaps the PENR1-binding site. It is also shown that binding of PENR1 induced bending of a DNA fragment spanning the PENR1-binding site between acvA and ipnA in vitro.