Synthesis of heme A from heme B (protoheme IX) most likely occurs in two steps with heme O as an intermediate. Bacillus subtilis CtaB, an integral membrane protein, functions in farnesylation of heme B to form heme O. CtaA, also a membrane protein, is required for heme A synthesis from heme O and appears to be a monooxygenase and/or a dehydrogenase. Wild-type ctaA and ctaB expressed together from plasmids in B. subtilis resulted in CtaA containing equimolar amounts of low-spin heme B and heme A; this form of CtaA was named cyt ba-CTA. A mutant ctaB gene was identified and characterised. It encodes a truncated CtaB polypeptide. Wild-type ctaA and the mutant ctaB gene on plasmids resulted in CtaA containing mainly low-spin heme B; this variant was named cyt b-CTA. The heme B component in cyt ba-CTA and cyt b-CTA showed identical properties; a mid-point redox potential of +85 mV, an EPR g(max) signal at 3.7, and a split alpha-band light absorption peak. The heme A component in cyt ba-CTA showed a mid-point potential of +242 mV, an EPR g(max) signal at 3.5, and the alpha-band light absorption peak at 585 nm. It is suggested that the CtaA protein contains two heme binding sites, one for heme B and one for substrate heme. The heme B would play a role in electron transfer, i.e. function as a cytochrome, in the monooxygenase and/or dehydrogenase reaction catalysed by CtaA whereas heme O/heme A would be substrate/product.