Structural organization of the bovine adrenodoxin gene

The gene structure of bovine adrenodoxin (Ad), a component of the steroid hydroxylating system in adrenal cortex mitochondria, was determined. This gene seems to be a single copy, but contains a pseudo exon in addition to the genuine gene region. The gene region for the active Ad is divided into 4 exons by 3 introns, and spans at least 27 kb. All the splice donor and acceptor sites follow the GT/AG rule. The transcription initiation site was determined by primer extension analysis, and putative TATA and GC boxes are present in the 5'-flanking region. A 12 base sequence, CCAGGGCCAGGG, is present 95 bases upstream from the transcription initiation site. A similar sequence was also found in the 5'-flanking regions of all genes for the steroid hydroxylating cytochromes P-450: P-450(SCC), P-450(11 beta), P-450(17 alpha), and P-450(C21), in the adrenal cortex. The consensus sequence for these five sequences is CGCAGGGCCATGGGA. The pseudo exon, exon 1', was present about 0.5 kb upstream from exon 1. It encodes the prepeptide region of the minor type Ad mRNA, but has a stop codon (TAA) in the frame. A TATA-box-like sequence and a CAAT-box-like sequence are also present in the 5'-flanking region of exon 1'. The GC content of exon 1 was over 80%, whereas for each of the other four exons, including exon 1', it was about 40%. Our observations suggest that exon 1, which encodes the prepeptide region of the major type Ad mRNA, has intruded into the region between exon 1' and exon 2.