Controlled expression of the transcriptional activator gene virG in Agrobacterium tumefaciens by using the Escherichia coli lac promoter

The Agrobacterium VirG protein is normally expressed from two promoters in response to multiple stimuli, including plant-released phenolics (at promoter P1) and acidic growth media (at promoter P2). To simplify the analysis of vir gene induction, we sought to create Agrobacterium strains in which virG could be expressed in a controllable fashion. To study the possibility of using the lac promoter and repressor, we constructed a plasmid containing the lac promoter fused to the lacZ structural gene. A derivative of this plasmid containing the lacIq gene was also constructed. The plasmid not containing lacIq expressed high levels of beta-galactosidase. The plasmid containing lacIq expressed beta-galactosidase at very low levels in the absence of o-nitrophenyl-beta-D-galactoside (IPTG) and at moderate levels in the presence of IPTG. We also fused the lac promoter to a virG::lacZ translational fusion and found that IPTG elevated expression of this translational fusion to moderate levels, though not to levels as high as from the stronger of the two native virG promoters. Finally, the lac promoter was used to express the native virG gene in strains containing a virB::lacZ translational fusion. virB expression in this strain depended on addition of IPTG as well as the vir gene inducer acetosyringone. In a similar strain lacking lacIq, virB expression was greater than in a strain in which virG was expressed from its native promoters. Expression of virG from the lac promoter did not alter the acidic pH optimum for vir gene induction, indicating that the previously observed requirement for acidic media was not due solely to the need to induce P2.