Cytochrome b is an electron transfer protein unique to neuroendocrine secretory vesicles. The Southern blot hybridization shows that it is a single copy gene highly conserved throughout phylogeny. The transcription unit spans approximately 11 kilobases, and heterologous transcription sites are located 404 bases 5` to the translation initiation codon. The sequence of the 5`-flanking region is GC-rich and lacks a typical TATA box at the usual position. However, it has a CAAT sequence at -132 and potential recognition sequences for several transcription factors including SP1, GR-PR-MMTV, AP4, gERE, JCV repeat, AP2, and NF-B. Each of the five transmembrane segments are encoded by five consecutive exons. This corroborates the five-transmembrane model proposed for human, mouse, and Xenopus rather than six proposed for bovine. The cytochrome was found to be highly expressed in colon cancer cell lines, T cell lymphomas, and K-562 cell lines. However, in B-cell lymphomas such as Burkitt's and Daudi, the cytochrome b expression was completely shut down. The results in this report are the first to demonstrate the structural organization and regulatory sequences of the cytochrome b gene encoding an integral membrane protein of neuroendocrine storage vesicles of neurotransmitters and peptide hormones. Unexpected results on cytochrome b expression in cells of lymphocytic origin and its complex regulation in tumor cells provide new insights into cytochrome b gene regulation.

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