Divergent transcription and a remote operator play a role in control of expression of a nopaline catabolism promoter in Agrobacterium tumefaciens.

The nocP-nocR divergent gene arrangement of the nopaline catabolism (noc) operon of the Agrobacterium tumefaciens Ti plasmid pTiT37 was examined with respect to the expression of the nocP promoter. Under repressive conditions, i.e. in the absence of nopaline, four distinct levels of PnocP expression were observed. The lowest level of expression, i.e. full repression, was detected in the presence of the NocR repressor, together with the remote noc operator and productive transcription from the divergent nocR promoter. The next level was observed in the absence of either the NocR protein or of the operator or of both. The third level was detected when abortive transcription from the nocR promoter occurred, irrespective of the presence or absence of the NocR protein. The highest level of PnocP expression was observed in the absence of both productive transcription from PnocR and the operator sequence, whether or not the NocR protein was present. Under inductive conditions, i.e. in the presence of nopaline, expression of PnocP was activated if both the NocR protein and the operator were present. Absence of either NocR or the operator resulted in lack of inducibility of the nocP promoter. Transcription from the divergent nocR promoter had no influence on the activation of PnocP. It was also found that the absence of the operator affected plasmid supercoiling in vivo. The results suggest that DNA topology has a role in the regulation of the nocP promoter.