Bacteria use the phoshorylation-dependent two-component paradigm to detect signals and to produce appropriate responses. They rely on these signaling pathways for adapting to fluctuating environmental conditions for optimal growth and survival. Sporulation in Bacillus subtillis is carried out under the control of a more sophisticated version of the two-component system called the phosphorelay. A unique component in this system is the phosphotransferase protein Spo0B. The site of phosphorylation in Spo0B is a histidine residue as in the histidine kinases. Additionally, Spo0B has structural resemblances to the autokinase domain of histidine kinases. These similarities suggest that Spo0B evolved from a histidine kinase with enhanced phosphotransferase activity. This manuscript examines the structural basis of evolution of Spo0B. The monomeric Spo0B comprises of two domains, an N-terminal helical hairpin followed by a C-terminal domain which folds like the ATPase domain of a histidine kinase. In the autokinase domain of a histidine kinase, the two sub-domains are connected by a long flexible linker. Spo0B appears to have evolved from an autokinase domain by deleting this long linker region and also by truncating a loop region from the ATPase domain necessary for ATP binding.
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