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.