ALTERED-FUNCTION MUTATIONS IN THE AGROBACTERIUM-TUMEFACIENS OCCR PROTEIN AND IN AN OCCR-REGULATED PROMOTER

OccR is a LysR-type transcriptional activator that controls the occQ a nd traR promoters of octopine-type Ti plasmids. The opine octopine con verts OccR from a repressor to an activator of occQ, shortens the prot ein's DNase I footprint, and decreases the angle of an OccR-caused DNA bend at the occQ promoter. In this study we first localized the cia-a cting DNA sequences required for regulated expression of occQ. To unde rstand better the mechanism of activation of OccR, we isolated mutatio ns both in the occQ promoter and in the occR gene which function diffe rently from the wild type. An occQ promoter mutation that changes the putative - 35 region of occQ from TTGACC to TTGACA increases the basal expression of occQ about 15-fold. Three mutations in occR were also i dentified, one of which activates occQ at fully constitutive levels in both the absence and presence of octopine. This mutation (E23G) is lo cated in the first helix of a putative helix-turn-helix DNA-binding mo tif. The other two occR mutations cause the protein to detect much low er concentrations of octopine than wild-type OccR protein does. These mutations (F113L and G148D) are located in a region of the protein tha t is predicted to contain the ligand-binding site.