Cytochrome c6 (Cyt c6) and Plastocyanin (Pc) are two well characterised redox carriers in the thylakoid lumen of cyanobacteria and algae. In cyanobacteria Cyt c6 or Pc shuttles electrons from the Cyt b6f complex to Photosystem I (PSI) or the terminal oxidase, and in plants Pc transfers electrons from the Cyt b6f complex to PSI. Along with Pc or Cyt c6, additional, poorly uncharacterized, low molecular weight electron carriers have recently been reported in cyanobacteria, algae and higher plants. These are Cyt cM and Cyt c6C/c6B in cyanobacteria and Cyt c6A in algae and higher plants.

The possible functions of these proteins were investigated using Arabidopsis strains in which the Cyt c6A gene was inactivated, and over-expression lines constructed in this work, as well as strains of the cyanobacterium Synechococcus elongatus PCC7942 in which the Cyt cM and Cyt c6C genes had been inactivated, also constructed in this work. The phenotypes of the modified organisms were assessed in different physiological conditions, particularly with regard to their ability to withstand low or high light levels. In addition, chlorophyll fluorescence studies were performed, and transcriptomic analysis was carried out for the cyanobacterial strains.

Impaired growth of the ΔCyt c6C strain was seen under extremely low light or extremely high light, and changes in levels of a large number of transcripts were detected in the mutant. Impaired growth of the ΔCyt cM strain was also seen under extremely low light levels. Changes in transcript levels in the mutant were also assessed. Cyt c6C and Cyt cM transcripts were up regulated under high light intensities. ΔCyt c6A Arabidopsis strains showed impaired growth at high light intensities, and strains overexpressing Cyt c6A showed impaired growth at low light intensities. Cyt c6A-EYFP fusion protein analysis by confocal microscopy indicated the localization of Cyt c6A fusion protein in the chloroplast, and possibly in the thylakoid lumen.

The findings suggested the presence of possible alternative electron pathways from the plastoquinone (PQ) pool to PSI and/or to the respiratory complexes. These alternative electron pathways assist the organisms to withstand extreme light conditions. It is proposed that in cyanobacteria Cyt c6C accepts electrons directly from the PQ pool and donates them to Pc or directly to PSI bypassing the Cyt b6f complex, and Cyt cM accepts electron directly from the PQ pool, donating them to cytochrome oxidase. In plants, Cyt c6A accepts electron directly from the PQ pool and donates them to Pc and thence to PSI or directly to PSI.