<p>This entry represents a haem-binding domain with a 4-helical bundle structure that is found in transmembrane di-haem cytochromes. The domain contains four transmembrane helices in an up-and-down bundle, and binds two haem groups in between the helices; three of the four haem-binding residues is conserved between family members. Proteins containing this domain include:</p><p> <ul><li>N-terminal domain of mitochondrial cytochrome b subunit, in which the domain contains an extra transmembrane linker helix that is absent in plant and cyanobacteria subunits [<cite idref="PUB00037404"/>].</li><li>Cytochrome b6 subunit of the cytochrome b6f complex, which provides the electronic connection between the photosystems I and II reaction centres of oxygenic photosynthesis, and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis [<cite idref="PUB00032072"/>].</li><li>Cytochrome gamma subunit of formate dehydrogenase-N (Fdn-N), which acts as a major component of <taxon tax_id="562">Escherichia coli</taxon> nitrate respiration [<cite idref="PUB00009871"/>].</li></ul> </p>