Signal structure of the cis-acting element recognized by VirG protein, a positive regulator in Agrobacterium

VirG protein is known to be the positive regulator for vir genes of Ri and Ti plasmids in Agrobacterium. To investigate the cis-acting element recognized by VirG, we have determined the transcriptional start points of the vir genes on Ri plasmid (pRiA4b). From the analysis of the upstream sequences, it has been found that the sequences generally similar to 5'TG(A/T)AA(C/T)3' appear in phase with an 11-base pair interval, and that the -35 and -10 regions of the promoters are located nearly in the reversal phase with those phasing sequences. These sequence characteristics are conserved in the vir genes on Ti plasmids (pTiA6 and pTiC58). Moreover, the similar sequence structures were found in the regulatory regions of the genes controlled by PhoB and OmpR, which are the E. coli positive regulators closely related to VirG in their amino-acid sequences.