Effects of the two-component sensor-regulator pairs DctBD and NtrBC upon the expression of a dctA::phoA fusion from Rhizobium meliloti were determined under excess and limiting nitrogen concentrations in Escherichia coli. Results indicated that NtrBC affected transcription from the dctA promoter on a number of regulatory levels and under different physiological conditions in the heterologous host. However, NtrBC-dependent cross-talk was not observed in free-living R. meliloti under the conditions tested. Comparisons of the predicted amino acid sequences of DctD and NtrC from various sources indicated a specific region of the NtrC from rhizobia, which may have diverged from a consensus NtrC/DctD sequence to minimise interference between the two component systems, NtrBC and DctBD.