Transcriptional induction of an Agrobacterium regulatory gene at tandem promoters by plant-released phenolic compounds, phosphate starvation, and acidic growth media

Transcription of the virG gene of Agrobacterium tumefaciens was previously shown to be expressed from two tandem promoters and to be responsive to three stimuli: plant-released phenolic compounds, phosphate starvation, and acidic media. In this report, I describe a set of deletions and other alterations of the 5' end of virG that show that the upstream promoter (P1) is necessary for induction by phenolic compounds and by phosphate starvation, whereas the downstream promoter (P2) is induced by acidic media. Upstream of promoter P1 there are three copies of a family of sequences (vir boxes) found near all VirA, VirG-inducible promoters. Site-directed mutagenesis of these sequences showed that vir box I and vir box III but not vir box II are needed for induction of P1 by acetosyringone. Induction of P1 by phosphate starvation requires vir box III (or an overlapping site), whereas vir box I and vir box II are not needed. The relative importance of promoters P1 and P2 in vir gene induction was tested by measuring the expression of a virB::lacZ fusion in strains containing mutations at either promoter P1 or P2. Mutations in either promoter significantly attenuated the expression of virB, indicating that both promoters play important roles in vir gene induction.