patD, a gene regulated by NtcA, is involved in the optimization of heterocyst frequency in the cyanobacterium Anabaena sp. PCC 7120

In the filamentous multicellular cyanobacterium Anabaena sp. PCC 7120, 5-10% of the cells differentiate into heterocysts, specialized in N2-fixation. Heterocysts and vegetative cells are mutually dependent for filament growth through nutrient exchange. Thus, the heterocyst frequency should be optimized to maintain the cellular carbon and nitrogen (C/N) balance for filament fitness in the environment. Here we report the identification of patD whose expression is directly activated in developing cells by the transcription factor NtcA. The inactivation of patD increases heterocyst frequency and promotes the up-regulation of the positive regulator of heterocyst development hetR, whereas its overexpression decreases heterocyst frequency. The change of heterocyst frequency resulting from the inactivation of patD leads to the reduction in competitiveness of the filaments under combined-nitrogen depleted conditions. These results indicate that patD regulates heterocyst frequency in Anabaena sp. PCC 7120, ensuring its optimal filament growth. IMPORTANCE Microorganisms have evolved various strategies in order to adapt to the environment and compete with other organisms. Heterocyst differentiation is a prokaryotic model for studying complex cellular regulation. The NtcA-regulated gene patD controls the ratio of heterocysts relative to vegetative cells on the filaments of Anabaena sp. PCC 7120. Such a regulation provides a mechanism through which carbon fixation by vegetative cells and nitrogen fixation by heterocysts are properly balanced, to ensure optimal growth and keep a competitive edge for long-term survival.