ThyD Is a Thylakoid Membrane Protein Influencing Cell Division and Acclimation to High Light in the Multicellular Cyanobacterium <i>Anabaena</i> sp. Strain PCC 7120

Cyanobacteria developed oxygenic photosynthesis and represent the phylogenetic ancestors of chloroplasts. The model strain Anabaena sp. strain PCC 7120 grows as filaments of communicating cells and can form heterocysts, cells specialized for N₂ fixation. In the Anabaena genome, ORF all2390 is annotated as encoding a SulA homolog, but sequence similarity to SulA of model bacteria is insignificant. We generated strains that lacked or overexpressed all2390, both of which showed instances of increased cell size, and observed that purified All2390 protein interfered with the in vitro polymerization of FtsZ. Heterocyst frequency diminished by all2390 inactivation and increased by all2390 overexpression. Overexpression retarded the dismantlement of Z-ring structures that determines commitment in the differentiating cells. Thus, All2390 can influence cell division affecting heterocyst differentiation. An All2390-GFP fusion protein localized to the thylakoid membranes including the honeycomb membranes, which harbor photosynthetic complexes, in the heterocyst polar regions. Notably, all2390 expression strongly increased under high light, conditions under which growth of the null mutant is compromised. Thus, All2390 appears essential for adaptation to high light conditions. We named All2390 ThyD to reflect its thylakoidal localization and its dual role in cell division dynamics and acclimation of thylakoid membranes to increased light intensity.