Transcription of Saccharomyces cerevisiae COX6, the nuclear gene for subunit VI of cytochrome c oxidase, is activated in heme-proficient cells, requires the HAP2 gene, and is subject to glucose repression. In this study, by deletion mutagenesis of the COX6 promoter, we identified two regions that are important for transcription. The first was an upstream activation site, UAS6. It was found to be contained within an 84-base-pair (bp) sequence, between bp -256 and -340 of the COX6 translational initiation codon, and to contain sequences required for activation by heme and HAP2 and for release from glucose repression. When located upstream of a CYC1-lacZ fusion gene, deleted for both of its UASs, this segment functioned as a UAS element. Although UAS6 could promote expression in either orientation, it showed a marked orientation dependence in its response to HAP2 and the carbon source. The second region lay between bp -255 and -91. It contained two of the three major 5' termini of COX6 mRNAs and a putative TATA box. Deletion analysis of this region demonstrated that the putative TATA box is not required for transcription and that this region is separable into two redundant domains.