Deeply Branching c₆-like Cytochromes of Cyanobacteria

Abstract

The cyanobacterium Synechococcus sp. PCC 7002 carries two genes, petJ1 and petJ2, for proteins related to soluble, cytochrome c6 electron transfer proteins. PetJ1 was purified from the cyanobacterium, and both cytochromes were expressed with heme incorporation in Escherichia coli. The expressed PetJ1 displayed spectral and biochemical properties virtually identical to those of PetJ1 from Synechococcus. PetJ1 is a typical cytochrome c6 but contains an unusual KDGSKSL insertion. PetJ2 isolated from E. coli exhibited absorbance spectra characteristic of cytochromes, although the alpha, beta, and gamma bands were red-shifted relative to those of PetJ1. Moreover, the surface electrostatic properties and redox midpoint potential of PetJ2 (pI 9.7; E(m,7) = 148 +/- 1.7 mV) differed substantially from those of PetJ1 (pI 3.8; E(m,7) = 319 +/- 1.6 mV). These data indicate that the PetJ2 cytochrome could not effectively replace PetJ1 as an electron acceptor for the cytochrome bf complex in photosynthesis. Phylogenetic comparisons against plant, algal, bacterial, and cyanobacterial genomes revealed two novel and widely distributed clusters of previously uncharacterized, cyanobacterial c 6-like cytochromes. PetJ2 belongs to a group that is distinct from both c6 cytochromes and the enigmatic chloroplast c 6A cytochromes. We tentatively designate the PetJ2 group as c6C cytochromes and the other new group as c6B cytochromes. Possible functions of these cytochromes are discussed.

CCS5, a Thioredoxin-like Protein Involved in the Assembly of Plastid c-Type Cytochromes

“…Note that in land plants like Arabidopsis, cytochrome f is the only c-type cytochrome that is required for photosynthesis. Cytochrome c 6 , the substitute for plastocyanin in copper-deficient conditions is not present in land plants but only in green algae and cyanobacteria (55,56). Based on the fact that the ccs5 mutant displays a dual deficiency in the assembly of plastid cytochromes c, namely cytochrome f and cytochrome c 6 , we have demonstrated the placement of CCS5/HCF164 in the cytochrome c assembly pathway (Figs.…”

The role of a disulfide bridge in the stability and folding kinetics of Arabidopsis thaliana cytochrome c6A

“…1A). This feature is not found in cyt c 6 , c 6B or c 6C and is thus unique to cyt c 6A members [5]. No conformational change in the disulfide bond or the LIP upon heme oxidation state change has been detected [10] and a number of hypotheses suggesting a catalytic role for this disulfide containing LIP to support possible cellular function have been put forward [11,12].…”

“…This function is possible due to cyt c 6 having a heme mid-point redox potential (E m ) in the region of +350 mV vs normal hydrogen electrode (NHE) which is thermodynamically favourable for electrons to be shuttled between the two membrane bound components. Recently, genomic and bioinformatic approaches have uncovered cyt c 6 -like proteins present in plants, green algae [2][3][4], and certain cyanobacteria [5], which are distinct from the photosynthetic cyt c 6 members. The plant cyt c 6 -like protein was the first of these to be discovered and was subsequently named cyt c 6A [4].…”

“…The plant cyt c 6 -like protein was the first of these to be discovered and was subsequently named cyt c 6A [4]. In keeping with this nomenclature, the cyt c 6 -like proteins identified in certain cyanobacteria have been named cyt c 6B and c 6C [5], but their functions have yet to be determined. However, for cyt c 6A and cyt c 6B current evidence confirms that they are likely to be functionally distinct from photosynthetic cyt c 6 due to their E m s being > 200 mV lower.…”

Cytochrome c 6-like protein as a putative donor of electrons to photosystem I in the cyanobacterium Nostoc sp. PCC 7119

“…3, all Cyt c 6 -like proteins constitute by themselves a Cyt c 6 9.7 9.0 ?335 12.1 9 10 7 11.4 9 10 6 1.7 9 10 5 Cyt c 6 -like 10.2 8.0 ?199 3.2 9 10 7 4.0 9 10 6 Pc 11.1 8.8 ?357 7.6 9 10 7 5.9 9 10 6 a k bim is the second-order rate constant for PSI reduction b k inf is the k bim extrapolated to infinite ionic strength c k et is the first-order electron transfer rate constant for PSI reduction different clade, as they are all in a single branch that is independent of the other heme proteins. Also, Bialek et al (2008) described two well-defined groups of cyanobacterial Cyt c 6 -like proteins, the Cyt c 6B and the Cyt c 6C . The Cyt c 6 -like protein described here falls into the Cyt c 6C group.…”