ANALYSIS OF THE ROS REPRESSOR OF AGROBACTERIUM VIRC AND VIRD OPERONS - MOLECULAR INTERCOMMUNICATION BETWEEN PLASMID AND CHROMOSOMAL GENES

The virulence genes of the Agrobacterium tumefaciens Ti plasmid are re gulated bo negatively. The products of the genes of the virC and virD operons play an important role in host specificity and T-DNA processin g. These operons are transcribed in opposite directions and therefore bear diametrically oriented promoters. These promoters are positively regulated by the VirG protein, which is believed to be activated throu gh phosphorylation by a histidine kinase encoded by the virA gene. The virC and virD operons are also regulated by a 15.5-kDa repressor prot ein encoded by the ros chromosomal gene. A mutation in ros causes the constitutive expression of virC and virD in the complete absence of th e VirG protein. It appears, therefore, that the Ros repressor interact s with the regulatory region of these operons. The Ros repressor is sh own here to bind to an upstream sequence (Ros box) comprising 40 bp be aring a 9-bp inverted repeat, TATATTTCA/TGTAATATA, in the promoter reg ion of these operons. The affinity for this sequence is specific and t enacious, since the addition of at least a 20,000-fold excess of compe titor DNA failed to remove the Ros protein coding sequence from the Ro s box. DNase I footprint analysis showed that the Ros box overlaps the binding site of VirG (Vir box). This result suggests that virC and vi rD transcription is modulated by Ros and VirG proteins.