DNA gyrase has no activity before subunits of GyrA and GyrB recombine to a tetrameric holoenzyme.A series of GyrB mutants has been constructed and the enzymatic activity of these GyrB mutants was analyzed to study the interaction domains of GyrB and GyrA subunits.Results showed that the C-terminus of GyrB subunit is the key domain to interact with GyrA subunit.Combined with analyzing GyrB dimensional structure,531-550 aa of GyrB was proposed to be the key domain of controlling enzymical activity of DNA gyrase and an ideal target for designing drug.