Biochemical complementation studies in vitro of gyrase subunits from different species

To investigate the functional equivalence of DNA gyrase subunits from different bacterial sources hybrid enzymes were formed using purified A and B subunits from three species of Streptomycetes, E. coli and B. subtilis. The activity of gyrase hybrids composed of heterologous gyr A and gyr B proteins and of the gyrases containing homologous subunits was characterized by binding studies and a cleavage assay with two different DNA fragments. Likewise the enzyme activity was monitored by the super-coiling and relaxation assay with pBR322 DNA. We found that cleavage reactions are largely determined by the source of the gyr B subunits whereas DNA supercoiling and relaxation reactions of pBR322 catalyzed by DNA gyrase are limited to a combination of homologous A and B subunits or of heterologous A and B subunits from the taxonomically related bacteria Streptomycetes.