A new class of signal transducer in His-Asp phosphorelay systems.

Nitrate transport activity of the LtnT permease of the cyanobacterium Synechococcus elongatus is activated when LtnA, a response regulator without an effector domain, is phosphorylated by LtnB, a hybrid histidine kinase. We identified a protein (LtnC) that is required for activation of LtnT. LtnC consists of an N-terminal histidine-containing phosphoacceptor (HisKA) domain, a receiver domain, and a unique C-terminal domain found in some cyanobacterial proteins. Because LtnC lacks an ATP-binding kinase domain of a histidine kinase, it is incapable of autophosphorylation, but LtnC is phosphorylated by LtnA. The histidine residue in the HisKA domain but not the aspartate residue in the receiver domain is essential for phosphorylation of LtnC and activation of LtnT. LtnC phosphorylation leads to oligomerization of the protein. Fusion of the C-terminal domain of LtnC to glutathione S-transferase, which forms oligomers, also activates LtnT, suggesting that oligomerization of the LtnC C-terminal domain causes LtnT activation. These results indicate that the C-terminal domain of LtnC acts as an effector domain that directs the output of the signal from the phosphorelay system. The two-step (His-Asp-His) phosphorelay system, composed of the LtnB, LtnA, and LtnC proteins, is distinct from the known phosphorelay systems, namely, the typical two-component system (His-Asp) and the multistep phosphorelay system (His-Asp-His-Asp), because the HisKA domain of LtnC is the terminal phosphoacceptor that determines the signal output. LtnC is a new class of signal transducer in His-Asp phosphorelay systems that contains a HisKA domain and an effector domain.