Deletion of the gene family of small chlorophyll-binding proteins (ScpABCDE) offsets C/N homeostasis in synechocystis PCC 6803

Small CAB-like proteins (SCPs) are single-helix light-harvesting-like proteins found in all organisms performing oxygenic photosynthesis. We investigated the function of these stress-induced proteins in the cyanobacterium Synechocystis sp. PCC 6803 by comparing a strain, which expresses SCPs constitutively, with a mutant lacking all five SCPs. In the absence of SCPs, cells were larger, and showed irregular thylakoid structure and cell-surfaces. Deletion of scp genes strongly affected the carbon-nitrogen balance, causing accumulation of carbohydrates and a decrease in N-rich compounds (proteins and chlorophyll a). Data from transcriptomic and metabolomic experiments demonstrated that SCPs mediated stabilization of chlorophyll a under stress conditions is crucial to maintain the C/N balance. SCPs diminished the formation of reactive oxygen species, preventing cellular damage by adjusting the de-novo production of chlorophyll a to demand levels. Lack of SCP expression under stress conditions had a large impact on the metabolism of the entire cell. Synechocystis 6803 PSI-less (Shen et al 1993) and PSI-less/ScpABCDE- (Xu et al 2004) mutants were cultivated at 30°C at a light intensity of 4–5 μmol photons m–2 s–1 in BG-11 growth medium (Rippka et al 1979) supplemented with 10 mM glucose. Cells (40–50 ml) from both control and the mutant cultures were collected at mid-log phase by rapid filtration (Pall Supor 800 Filter, 0.8 mm) and total RNA extracted after Pinto et al. (Pinto et al 2009). Single-stranded cDNA preparation, labelling, and hybridization onto the microarray slides were performed after Eisenhut et al. (2007). The microarray slides were scanned with an Agilent Microarray Scanner (model G2505B) with default settings.