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The metabolic flexibility of cyanobacteria has enabled them to survive and flourish under a wide range of environmental conditions. In particular their ability to perform oxygenic photosynthesis and respiration within the same cell has allowed them to generate ATP from diverse energy sources. While great strides have been made in understanding the molecular details of the photosynthetic apparatus in cyanobacteria, much less is known about the protein complexes involved in respiration. Here we review our current knowledge of cyanobacterial NDH-1 complexes. which are related to the type-I NADH dehydrogenase (NADH:quinone oxidoreductase) found in mitochondria and in other types of bacteria. The picture that is emerging from a combination of molecular biology, biochemistry, proteomics and structural biology is that cyanobacteria synthesize different types of NDH-1 complex depending on the physiological needs of the cell, and that cyanobacterial NDH-1 complexes play important physiological roles not only in respiration but also in cyclic electron flow around Photosystem I in the light and in the uptake of CO2 into the cell. These differences in function are reflected by differences in the subunit composition of the different types of NDH-1 complex. The subunit compositions of the cyanobacterial NDH-1 complexes and the closely related Ndh (or NDH) complex in chloroplasts suggest that they form a separate sub-class of the complex I family of enzymes. However, there are important differences between the NDH-1 class of complexes in chloroplasts and cyanobacteria, which appear to reflect adaptations that have occurred since their evolutionary divergence.

Cyanobacteria possess a complex CO2-concentrating mechanism (CCM), which makes them survive and acclimate in a variety of CO2 environments. The efficiency of CCM is enhanced under low inorganic carbon environments by expression of a set of protein complexes. The inducible CCM genes are regulated by LysR family proteins NdhR and CmpR. To investigate the involvement of proteases in induction of the CCM complexes, we studied the FtsH2 protease mutant of Synechocystis sp. PCC 6803. The cells were grown under high CO2 and then shifted to low CO2, followed by a proteome analysis of the membrane protein complexes and RT-PCR. Interestingly, the inducible CCM complexes were not detected in the FtsH2 mutant upon shift to low CO2. The transcripts of the inducible CCM genes and their regulator ndhR failed to accumulate, indicating that the regulation by the FtsH2 protease is upstream of NdhR. Moreover, functional photosynthesis was shown a prerequisite for induction of CCM in WT at low CO2, possibly via generation of oxidative stress, which was shown here to transiently enhance the expression of inducible CCM genes even at high CO2 conditions.