, a Gene Regulated by NtcA, Is Involved in the Optimization of Heterocyst Frequency in the Cyanobacterium sp. Strain PCC 7120.

In the filamentous multicellular cyanobacterium sp. strain PCC 7120, 5 to 10% of the cells differentiate into heterocysts, which are specialized in N 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 , whose expression is directly activated in developing cells by the transcription factor NtcA. The inactivation of increases heterocyst frequency and promotes the upregulation of the positive regulator of heterocyst development , whereas its overexpression decreases the heterocyst frequency. The change in heterocyst frequency resulting from the inactivation of leads to the reduction in competitiveness of the filaments under combined-nitrogen-depleted conditions. These results indicate that regulates heterocyst frequency in sp. PCC 7120, ensuring its optimal filament growth. 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 controls the ratio of heterocysts relative to vegetative cells on the filaments of sp. strain 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.