A Phosphorylation-Dependent Partner-Switching-Like Module Regulates a Glycosyltransferase Required for Heterocyst Polysaccharide Layer Formation in Anabaena sp. Strain PCC 7120.

Heterocyst-specific polysaccharide (Hep) is an essential component of the heterocyst envelope in the filamentous cyanobacterium Anabaena sp. PCC 7120; however, the regulatory mechanisms that control Hep layer formation remain poorly understood. Herein, we reveal that Hep layer formation is regulated by a phosphorylation-dependent partner-switching-like system involving the phosphatase HenR and the STAS domain-containing glycosyltransferase All4160. A nonphosphorylatable All4160 variant restores Hep formation in a henR disruptant, indicating that phosphorylation negatively regulates All4160 function. Using a bacterial two-hybrid assay, we identified two candidate kinases that interact with All4160, that is, Alr3423 and All2284, which phosphorylate All4160 in vitro. Genetic analysis revealed that deletion of alr3423, not all2284, suppresses the defect of the henR disruptant, indicating that Alr3423 plays a key role in regulating All4160 in vivo. This study identifies a phosphorylation switch that controls Hep layer formation and reveals a regulatory mechanism acting directly on a polysaccharide biosynthetic enzyme.