We describe the solution structures of two- and three-domain constructs of the sensor histidine kinase PrrB from Mycobacterium tuberculosis, which allow us to locate the HAMP linker relative to the ATP binding and dimerization domains. We show that the threedomain construct is active both for autophosphorylation and for phosphotransfer to the cognate response regulator, PrrA. We also describe the high-resolution crystal structure of the catalytic domain alone, and we show that, in solution, it binds ATP. The conformational flexibility of this domain is discussed and related to other structural information.