TI PLASMID-ENCODED OCTOPINE AND NOPALINE CATABOLISM IN AGROBACTERIUM - SPECIFICITIES OF THE LYSR-TYPE REGULATORS OCCR AND NOCR, AND PROTEIN-INDUCED DNA BENDING

The occ and noc regions in octopine and nopaline Ti plasmids, respecti vely, are responsible for the catabolism of octopine and nopaline in A grobacterium. The functions are activated in the presence of the opine s by OccR and NocR, two related regulatory proteins, and the promoters contain common sequence motifs. We have investigated heterologous int eractions between the regulators and the promoters. Previous experimen ts using all possible heterologous combinations of opines, regulators, and promoters in vivo had demonstrated that only the combination of n opaline, NocR, and the occ promoter led to limited promoter activation . We now show that OccR and NocR bind to the heterologous promoters in vitro and in vivo. The weak or non-existent promoter activation actua lly observed could be explained by the assumption that OccR and NocR u se different activation mechanisms; we investigated protein-induced DN A bending because of reports that the two regulators differ in this re spect. Analysis with a bending vector showed that both OccR and NocR i nduced a DNA bend that is relaxed in the presence of the respective op ine. The data suggest that subtle differences in regulator/promoter in teractions are responsible for the inactivity of the heterologous comb inations. Investigations with a chimeric NocR/OccR protein indicated t hat it induced a DNA bend in both promoters. No opine-induced relaxati on was detectable with the hybrid, and the inducible promoter was not activated. These findings suggest that bend relaxation may be an integ ral part of promoter activation.