Formation of several bacterial c-type cytochromes requires a novel membrane-anchored protein that faces the periplasm.

We report here the discovery of a novel bacterial gene (cycH) whose product is involved in the biogenesis of most of the cellular cytochromes c. The cycH gene was detected in the course of characterizing a cytochrome oxidase-deficient Bradyrhizobium japonicum Tn5 mutant (strain COX3) in which the transposon insertion disrupted cycH. All of the c-type cytochromes detectable in aerobically grown B. japonicum wild-type cells were absent in the COX3 mutant, with the exception of cytochrome c1. A secondary phenotypic effect was the spectroscopic absence of the aa3-type cytochrome c oxidase. The nucleotide sequence of the cloned wild-type cycH gene predicted a membrane-bound 369-amino-acid protein with an M(r) of 39727. Results from studies on its membrane topology suggested that approximately 110 N-terminal amino acids are involved in anchoring the protein in the membrane, whereas the remaining two-thirds of the protein are exposed to the periplasm. We postulate that the CycH protein plays an essential role in an as yet unidentified periplasmic step in the biogenesis of holocytochromes c, except that of cytochrome c1.