CHARACTERIZATION OF THE GLNK-AMTB OPERON AND THE INVOLVEMENT OF AMTB IN METHYLAMMONIUM UPTAKE IN AZORHIZOBIUM-CAULINODANS

This work reports the characterisation of the Azorhizobium caulinodans amtB gene, the deduced protein sequence of which shares similarity to those of several ammonium transporters. amtB is located downstream fr om glnK, a glnB-like gene. It is cotranscribed with glnK from an NtrC- and sigma(54)-dependent promoter. glnK and amtB insertion mutant stra ins have been isolated. Methylammonium uptake was assayed in these str ains and in other mutant strains in which the regulation of nitrogen m etabolism is impaired. Our data suggest that the AmtB protein is an am monium transporter, which is mainly regulated by NtrC in response to n itrogen availability.