Nopaline causes a conformational change in the NocR regulatory protein-nocR promoter complex of Agrobacterium tumefaciens Ti plasmid pTiT37.

The nocR gene of Agrobacterium tumefaciens Ti plasmid pTiT37 is the regulatory gene of the nopaline catabolism (noc) operon of pTiT37. We have cloned and sequenced nocR, which encodes a DNA-binding protein. The deduced amino acid sequence is similar to those of members of the LysR family of prokaryotic activator proteins. Gel retardation experiments demonstrated that the NocR protein binds to the nocR promoter in both the presence and absence of nopaline. The increased mobility of the complex and alterations in the DNase I footprints revealed a nopaline-induced conformational change in the NocR-DNA complex. Sequence analysis of the NocR binding site indicated the presence immediately downstream of the -10 sequence of the nocR promoter of a 12 bp putative operator overlapping a consensus gyrase recognition sequence and an 18 bp long alternating purine-pyrimidine sequence. These results suggest that nopaline-induced alterations in the NocR protein-nocR promoter complex might control gene expression in the noc operon.