Regulation of the virC and virD promoters of pTiC58 by the ros chromosomal mutation of Agrobacterium tumefaciens.

The virC and virD operons of the virulence region of the Ti plasmid are highly regulated, requiring a transcriptional regulator that is encoded by virG and is activated by the product of virA and plant phenolics such as acetosyringone. Full expression of virC and virD of octopine and nopaline Ti plasmids is also obtained by a mutation in the ros gene of the Agrobacterium tumefaciens chromosome. S1-nuclease analysis, in vitro transcription, and DNase I protection experiments with A. tumefaciens RNA polymerase revealed virD promoters tandemly arranged, both of which are functional in the Ros mutant, while only one is functional in the presence of acetosyringone. A third (overlapping) promoter appears to be responsible for transcription of virC. Expression of virC and virD appears to be modulated by factors within the bacterium by means of a mechanism that is independent of factors produced during infection of the host plant.