Cytochrome c_M from Synechocystis 6803

Abstract

Based on DNA sequence data a novel c-type cytochrome, cytochrome c M , has been predicted to exist in the cyanobacterium Synechocystis 6803. The precursor protein consists of 105 amino acids with a characteristic heme-binding motif and a hydrophobic domain located at the N-terminal end that is proposed to act as either a signal peptide or a membrane anchor. For the first time we report the detection of cytochrome c M in Synechocystis 6803 using Western blot analysis. The soluble portion cytochrome c M has been overexpressed in Escherichia coli in two forms, one with a poly histidine tag to facilitate purification and one without such a tag. The overexpressed protein has been purified and shown to bind heme, exhibiting an absorption peak in the Soret band near 416 nm and a peak in the a band at 550 nm. The extinction coefficient of cytochrome c M is 23.2^0.5 mm 21´c m 21 for the reduced minus oxidized a band peak (550±535 nm). The isoelectric point of cytochrome c M is 5.6 (without the histidine tag), which is significantly lower than the pI of 7.2 predicted from the amino acid sequence. The redox midpoint potential of cytochrome c M expressed in E. coli is 151^5 mV (pH 7.1), which is quite low compared to other c-type cytochromes in which a histidine and a methionine residue serve as the axial ligands to the heme. This work opens the way for determining the three-dimensional structure of cytochrome c M and investigating its function in cyanobacteria.

A comparative structural and functional analysis of cytochrome cM, cytochrome c6 and plastocyanin from the cyanobacterium Synechocystis sp. PCC 6803

“…As shown in Table 1 , the molecular mass of Cyt c M deduced from the gene sequence (8.3 kDa) and that calculated by SDS–PAGE (7.9 kDa) is similar to those reported for Synechocystis Cyt c 6 and Pc [7,23]. The midpoint redox potential value of Cyt c M determined in this work (+150 mV, at pH 7) is very similar to that previously described for the modified Cyt c M [2], but significantly lower than that reported for Cyt c 6 and Pc (+350 mV, at pH 7) [23,24].…”

Unexpected changes in photosystem I function in a cytochrome c6-deficient mutant of the cyanobacterium Synechocystis PCC 6803

“…The physiological data presented above indicated that electron transfer to PSI could take place in the absence of [17] have revealed in vitro a redox potential that would be too low for such a function in our present understanding of electron transfer from plastoquinone (PQ) to the PSI reaction centre. The question on how electrons transfer to PSI in petJ mutant grown in the absence of copper ion remains to be answered in detail.…”

Terminal oxidases of cyanobacteria

“…Although studies with soluble CtaC indicate that Cyt c M is not absolutely required for electron transfer [18][19][20], it may promote binding or act as a redox mediator between soluble electron carriers and COX [26]. Although the measured redox potential (E m ) of soluble Cyt c M was only 150 mV [27] (which would raise a problem for its reduction by Cyt f (cytochrome f ), E m ≈ 330 mV), membrane localization and/or the presence of other COX subunits could potentially increase this to allow Cyt c M to act as a redox mediator.…”