The cytochrome bc1 complex of the respirator chain and the b6 f complex of OXYGENIC photosynthesis have long been known (Hauska et al., 1983) to be similar in (a) their redox prosthetic groups, which are present in the same stoichiometry, two b- and one c-type heme, one [2Fe-2S] cluster per unit; (b) use of quinol/semiquinol as electron and proton donor; and (c) their functions:
(i) mediation of electron transfer between the major integral electron donor (dehydrogenases and photosystem II, respectively) an acceptor (cytochrome oxidase and photosystem I) complexes
(ii) electrogenic charge transfer and H+ translocation (Soriano et al., 1999).

But there are several important differences between bc1 and b6f (Soriano et al., 1999) as well:

1. The cyt b (bc1) polypeptide with eight TM helices A-H is replaced in the b6 f complex by two smaller integral polypeptides (cyt b6 and subunit IV) consisting of helices A-D and E-G (Widger et al., 1984).

2. Cytochromes f and c1 are completely different in terms of both proteins sequence and structure, except for both being c-type cytochromes with the characteristic Cys-X-Y-Cys-His c-heme binding sequence motif.

3. While the four larger subunits of the b6 f complex have structural and/or functional similarity to that of bc1, the 3(4) small subunits are unique to the b6 f complex.

4. There are one molecule each of chlorophyll a (Huang et al., 1994) and beta-carotene (Zhang et al., 1999a) present in the b6 f complex (absent in bc1).

5. Lipid content can be different, as there are 5 molecules per monomer of the unique lipid monogalactosyl-diacylglycerol present in the b6 f complex (Soriano et al., 1999).

The bc1 complexes are encoded in Subsystem:Ubiquinone_Menaquinone-cytochrome_c_reductase_complexes.

References:

1. G.M. Soriano, M.V. Ponamarev, C.J. Carrell, D. Xia, J.L. Smith, and W.A. Cramer. 1999. Comparison of the Cytochrome bc1 Complex with the Anticipated Structure of the Cytochrome b6 f Complex: De Plus Ca Change de Plus C’est la Meme Chose. Journal of Bioenergetics and Biomembranes, 31(3):201-213

Observations, conjectures:

1. Missing small subunits -
Cytochrome b6-f complex of cyanobacteria is formed by four main relatively large subnits (PetA, PetB, PetC, and PetD) and several small hydrophobic 3.2-4.2 kDa polypeptides PetG, L, M, and N. In addition to these subunits, plant cyt b6/f complex contains PetH (PMID: 11483610). While orthologs of large conserved subunits are easily identifiable in a genome sequence, the small subunits are often overlooked during ORF calling due to their small size. Hence, the absence of orthologs of small subunits in a genome does NOT imply nonfunctional cyt b6-f complex.

2. PetC protein family -
Cyanobacteria come in two cytocrome b6-f flavors: some [variant code 1] contain a single Rieske iron sulfur protein (encoded by petC1 always cotranscribed with petA). Others [variant code 2] contain from 2 to 4 additional Rieske proteins (encoded by genes petC2, petC3, etc., differing from petC1 to various degrees). So far, association of PetC2 and PetC3 with functional cytocrome b6-f complex has been demonstrated formally only in Synechocystis PCC 6803, and merely inferred in other variant 2 species.

3. A speculation/prediction:
While biological significance of the presence of different Rieske proteins is not clear, I can’t help noting, that ALL “mono-PetC” species identified so far are strict photoautptrophs, while all (but one – see below) “multiple-PetC” species are capable of photoheterotrophic growth (inferior to photoautrophy in a life of a cyanobacterium). Since cytochrome b6/f complex in cyanobacteria is an essential component of both the respiratory and photosynthetic electron transfer chain, can it be that substitution of the default “photoautotrophic” PetC1 Rieske protein for a different one in the cyt b6/f complex is a necessary requirement for activation of (photo)heterotrophic growth based solely on respiratory electron chain?

Nostoc (Anabaena) sp. PCC 7120 – the only exception of this observation – is an interesting case in itself. Comparative genomic analysis of central carbon metabolism in cyanobacteria (conducted earlier by Dr. Ivanova et al.) has detected consistent differences between obligate and facultative photoautotrophs in organization of glycolysis and TCA. Interestingly, Nostoc sp. PCC 7120 was an exception in that study as well: it was the only photoautotrophic cyanobacterium with “photoheterotrophic” type of its central carbon metabolism.
If this observation happens to be true, the presence of PetC protein family in a newly sequenced cyanobacterial genome along with specific features of central carbon metabolism could be used for prediction of its growth capabilities.