Two-component regulatory systems responsive to environmental stimuli share strongly conserved domains with the nitrogen assimilation regulatory genes ntrB and ntrC.

We report that the ntrB and ntrC proteins of Bradyrhizobium sp. [Parasponia] strain RP501 share homology with other regulatory proteins. There is extensive conservation of C-terminal regions between products of RP501 ntrB; Klebsiella pneumoniae ntrB; Escherichia coli envZ, cpxA, and phoR; Agrobacterium tumefaciens virA; and, to a lesser extent, E. coli cheA. There is also extensive conservation of N-terminal regions between products of RP501 ntrC; K. pneumoniae ntrC; E. coli ompR, sfrA, phoB, cheY and cheB; Salmonella typhimurium cheB and cheY; Bacillus subtilis spoOA and spoOF; and A. tumefaciens virG. We propose that these regulatory genes comprise two-component regulatory systems that evolved from a common ancestral system that involved transduction of information about the status of the environment by one protein domain (the C-terminal regions conserved among ntrB, envZ, etc.) to a second one (the N-terminal region conserved among ntrC, ompR, etc.). The ntrC-set protein then acts upon a specific responding mechanism, typically as a transcriptional activator but also as an effector of the maturation of outer membrane proteins or as a modulator of the direction of flagella rotation.