NDH-1L with a truncated NdhM subunit is unstable under stress conditions in cyanobacteria

Cyanobacterial NdhM, an oxygenic photosynthesis-specific NDH-1 subunit, has been found to be essential for the formation of a large complex of NDH-1 (NDH-1L). The cryo-electron microscopic (cryo-EM) structure of NdhM from Thermosynechococcus elongatus showed that the N-terminus of NdhM contains three β-sheets, while two α-helixes are present in the middle and C-terminal part of NdhM. Here, we obtained a mutant of the unicellular cyanobacterium Synechocystis 6803 expressing a C-terminal truncated NdhM subunit designated NdhMΔC. Accumulation and activity of NDH-1 were not affected in NdhMΔC under normal growth conditions. However, the NDH-1 complex with truncated NdhM is unstable under stress. Immunoblot analyses showed that the assembly process of the cyanobacterial NDH-1L hydrophilic arm was not affected in the NdhMΔC mutant even under high temperature. Thus, our results indicate that NdhM can bind to the NDH-1 complex without its C-terminal α-helix, but the interaction is weakened. NDH-1L with truncated NdhM is more prone to dissociation, and this is particularly evident under stress conditions.