Comparative Targeted Proteomics of the Central Metabolism and Photosystems in SigE Mutant Strains of Synechocystis sp. PCC 6803

A targeted proteome analysis was conducted to investigate the SigE dependent-regulation of central metabolism in Synechocystis sp. PCC 6803 by directly comparing the protein abundance profiles among the wild type, a sigE deletion mutant (ΔsigE), and a sigE over-expression (sigEox) strains. Expression levels of 112 target proteins, including the central metabolism related-enzymes and the subunits of the photosystems, were determined by quantifying the tryptic peptides in the multiple reaction monitoring (MRM) mode of liquid-chromatography–triple quadrupole mass spectrometry (LC–MS/MS). Comparison with gene-expression data showed that although the abundance of Gnd protein was closely correlated with that of gnd mRNA, there were poor correlations for GdhA/gdhA and glycogen degradation-related genes such as GlgX/glgX and GlgP/glgP pairs. These results suggested that the regulation of protein translation and degradation played a role in regulating protein abundance. The protein abundance profile suggested that SigE overexpression reduced the proteins involved in photosynthesis and increased GdhA abundance, which is involved in the nitrogen assimilation pathway using NADPH. The results obtained in this study successfully demonstrated that targeted proteome analysis enables direct comparison of the abundance of central metabolism- and photosystem-related proteins.