cyAbrB Transcriptional Regulators as Safety Devices To Inhibit Heterocyst Differentiation in sp. Strain PCC 7120.

Cyanobacteria are monophyletic organisms that perform oxygenic photosynthesis. While they exhibit great diversity, they have a common set of genes. However, the essentiality of them for viability has hampered the elucidation of their functions. One example of these genes is (also known as in sp. strain PCC 7120), encoding a transcriptional regulator. In the present study, we investigated the function of in the heterocyst-forming cyanobacterium sp. PCC 7120 through CRISPR interference, a method that we recently utilized for the photosynthetic production of a useful chemical in this strain. Conditional knockdown of in the presence of nitrate resulted in the formation of heterocysts. Two genes, and , which are required for heterocyst formation, were upregulated by knockdown in the presence of combined nitrogen sources. These genes are known to be induced by HetR, a master regulator of heterocyst formation. was not induced by knockdown. and were repressed by direct binding of CalA/cyAbrB1 to their promoter regions in a HetR-independent manner. In addition, the overexpression of abolished heterocyst formation upon nitrogen depletion. Also, knockout of / (a paralogue gene of ), in addition to knockdown of , enhanced heterocyst formation in the presence of nitrate, suggesting functional redundancy of cyAbrB proteins. We propose that a balance between amounts of HetR and CalA/cyAbrB1 is a key factor influencing heterocyst differentiation during nitrogen stepdown. We concluded that cyAbrB proteins are essential safety devices that inhibit heterocyst differentiation. Spore formation in and has been extensively studied as models of prokaryotic nonterminal cell differentiation. In these organisms, many cells/hyphae differentiate simultaneously, which is governed by a network in which one regulator stands at the top. Differentiation of heterocysts in sp. strain PCC 7120 is unique because it is terminal, and only 5 to 10% of vegetative cells differentiate into heterocysts. In this study, we identified CalA/cyAbrB1 as a repressor of two genes that are essential for heterocyst formation independently of HetR, a master activator for heterocyst differentiation. This finding is reasonable for unique cell differentiation of because CalA/cyAbrB1 could suppress heterocyst differentiation tightly in vegetative cells, while only cells in which HetR is overexpressed could differentiate into heterocysts.