A working model of the transcriptional regulation and function of NfdnaK2 in protecting PSII against drought stress in the subaerial cyanobacterium N. flagelliforme. The steel blue wavy symbol represents the upstream region of the NfdnaK2 open reading frame (steel blue arrow region), and other symbols are as labeled. CP47, CP43, D1, and D2 proteins are the PSII core subunits, and green and red rectangles labeled by D1 protein represent functional and damaged D1 protein respectively. OEC, Oxygen evolution complex of PSII.