PetC1 Is the Major Rieske Iron-Sulfur Protein in the Cytochrome b6f Complex of Synechocystis sp. PCC 6803

Abstract

Many of the completely sequenced cyanobacterial genomes contain a gene family that encodes for putative Rieske iron-sulfur proteins. The Rieske protein is one of the large subunits of the cytochrome bc-type complexes involved in respiratory and photosynthetic electron transfer. In contrast to all other subunits of this complex that are encoded by single genes, the genome of the cyanobacterium Synechocystis PCC 6803 contains three petC genes, all encoding potential Rieske subunits. Most interestingly, any of the petC genes can be deleted individually without altering the Synechocystis phenotype dramatically. In contrast, double deletion experiments revealed that petC1 and petC2 cannot be deleted in combination, whereas petC3 can be deleted together with any of the other two petC genes. Further results suggest a different physiological function for each of the Rieske proteins. Whereas PetC2 can partly replace the dominating Rieske isoform PetC1, PetC3 is unable to functionally replace either PetC1 or PetC2 and may have a special function involving a special donor with a lower redox potential than plastoquinone. A predominant role of PetC1, which is (partly) different from PetC2, is suggested by the mutational analysis and a detailed characterization of the electron transfer reactions in the mutant strains.

Multiple Rieske Proteins Enable Short- and Long-term Light Adaptation of Synechocystis sp. PCC 6803

“…2A): whereas CydA was essentially undetectable by immunoblot analysis in WT, ⌬petC2, and ⌬petC3 strains, its expression was dramatically increased in the ⌬petC1 strain. This is in good agreement with earlier observations of a significantly increased cytochrome-bd oxidase activity in the ⌬petC1 strain (40). Most likely, due to cytochrome b 6 f complex deficiency in the ⌬petC1 strain, cytochrome-bd oxidase has to partly take over the role of the cytochrome b 6 f complex in PQH 2 re-oxidation to keep electron transport and energy transduction efficient, i.e.…”

“…Usually, the construction of knock-out strains is a highly valuable standard method to elucidate the potential in vivo function of such proteins. The generation of ⌬petC Synechocystis strains has been reported recently (40), and it has already been shown that electron transfer is modified at least in the ⌬petC1 strain and that PetC2 can partly replace the major Rieske protein PetC1. However, it is unknown whether the absence of PetC1 (or the other PetC isoforms) influences pigment compositions in the cells or affects the expression, accumulation, or stability of the cytochrome b 6 f complex.…”

“…2A) indicating a defect in formation of functional cytochrome b 6 f complexes upon deletion of petC1. As an increase in the cytochrome-bd oxidase activity in the ⌬petC1 strain had already been observed (35,40), we also tested the expression of the major cytochrome-bd oxidase subunit I (CydA) both in WT and ⌬petC strains ( Fig. 2A): whereas CydA was essentially undetectable by immunoblot analysis in WT, ⌬petC2, and ⌬petC3 strains, its expression was dramatically increased in the ⌬petC1 strain.…”

“…40. Synechocystis WT and mutant strains were grown in BG-11 medium (42) at 30°C in glass tubes bubbled with 5% CO 2 -enriched air.…”

“…Although PetC1 and PetC2 show high sequence identity, PetC3 differs significantly. In vivo analysis of petC gene-deficient mutant strains have indicated that PetC1 is the major Rieske iron-sulfur protein in Synechocystis PCC 6803 (40). Moreover, PetC3 has a much lower midpoint potential (ϩ135 mV) than PetC1 and PetC2 (300 -320 mV), suggesting that different electron donors have to reduce PetC3 (41).…”