Heterocysts are terminally differentiated cells of filamentous cyanobacteria, which are specialized for nitrogen fixation. Because nitrogenase is easily inactivated by oxygen, the intracellular environment of heterocysts is kept microoxic. In heterocysts, the oxygen-evolving photosystem II is inactivated, a heterocyst-specific envelope with an outer polysaccharide layer and an inner glycolipid layer is formed to limit oxygen entry, and oxygen consumption is activated. Heterocyst differentiation, which is accompanied by drastic morphological and physiological changes, requires strictly controlled gene expression systems. Here, we investigated the functions of a CRP-family transcriptional regulator, DevH, in the process of heterocyst differentiation. A devH-knockdown strain, devH-kd, was created by replacing the original promoter with the gifA promoter, which is repressed during heterocyst differentiation. Although devH-kd formed morphologically distinct cells with the heterocyst envelope polysaccharide layer, it was unable to grow diazotrophically. Genes involved in construction of the microoxic environment, such as cox operons and the hgl island, were not upregulated in devH-kd. Moreover, expression of the nif gene cluster was completely abolished. Although CnfR was expressed in devH-kd, the nif gene cluster was not induced even under microoxic conditions. Thus, DevH is necessary for the establishment of a microoxic environment and induction of nitrogenase in heterocysts.
Keywords: Anabaena; heterocyst differentiation; microoxic intracellular environment; nitrogenase; transcriptional regulation.
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