Integrated Transcriptomic and Metabolomic Characterization of the Low-Carbon Response Using anndhRMutant ofSynechocystissp. PCC 6803

The acquisition and assimilation of inorganic carbon (Ci) represents the largest flux of inorganic matter in photosynthetic organisms; hence, this process is tightly regulated. We examined the Ci-dependent transcriptional and metabolic regulation in wild-typeSynechocystissp. PCC 6803 compared with a mutant defective in the main transcriptional repressor for Ci acquisition genes, the NAD(P)H dehydrogenase transcriptional regulator NdhR. The analysis revealed that many protein-coding transcripts that are normally repressed in the presence of high CO₂ (HC) concentrations were strongly expressed inΔndhR, whereas other messenger RNAs were strongly down-regulated in mutant cells, suggesting a potential activating role for NdhR. A conserved NdhR-binding motif was identified in the promoters of derepressed genes. Interestingly, the expression of some NdhR-regulated genes remained further inducible under low-CO₂ conditions, indicating the involvement of additional NdhR-independent Ci-regulatory mechanisms. Intriguingly, we also observed that the abundance of 52 antisense RNAs and 34 potential noncoding RNAs was affected by Ci supply, although most of these molecules were not regulated through NdhR. Thus, antisense and noncoding RNAs could contribute to NdhR-independent carbon regulation. In contrast to the transcriptome, the metabolome inΔndhRcells was similar to that of wild-type cells under HC conditions. This observation and the delayed metabolic responses to the low-CO₂ shift inΔndhR, specifically the lack of transient increases in the photorespiratory pathway intermediates 2-phosphoglycolate, glycolate, and glycine, suggest that the deregulation of gene expression in theΔndhRmutant successfully preacclimates cyanobacterial cells to lowered Ci supply under HC conditions.