Transcriptional Activation of Virulence Genes of Rhizobium etli

Recently,Rhizobium etli , in addition toAgrobacterium spp., has emerged as a prokaryotic species whose genome encodes a functional machinery for DNA transfer to plant cells. To understand thisR. etli -mediated genetic transformation, it would be useful to define how itsvir genes respond to the host plants. Here, we explored the transcriptional activation of thevir genes contained on theR. etli p42a plasmid. Using a reporter construct harboringlacZ under the control of theR. etli virE promoter, we show that the signal phenolic molecule acetosyringone (AS) inducesR. etli vir gene expression both in anR. etli background and in anAgrobacterium tumefaciens background. Furthermore, in both bacterial backgrounds, the p42a plasmid also promoted plant genetic transformation with a reporter transfer DNA (T-DNA). Importantly, theR. etli vir genes were transcriptionally activated by AS in a bacterial species-specific fashion in regard to the VirA/VirG signal sensor system, and this activation was induced by signals from the natural host species of this bacterium but not from nonhost plants. The early kinetics of transcriptional activation of the majorvir genes ofR. etli also revealed several features distinct from those known forA. tumefaciens : the expression of thevirG gene reached saturation relatively quickly, andvirB2 , which inR. etli is located outside thevirB operon, was expressed only at low levels and did not respond to AS. These differences invir gene transcription may contribute to the lower efficiency of T-DNA transfer ofR. etli p42a than of T-DNA transfer of pTiC58 ofA. tumefaciens .IMPORTANCE The region encoding homologs ofAgrobacterium tumefaciens virulence genes in theRhizobium etli CE3 p42a plasmid was the first endogenous virulence system encoded by the genome of a non-Agrobacterium species demonstrated to be functional in DNA transfer and stable integration into the plant cell genome. In this study, we explored the transcriptional regulation and induction of virulence genes inR. etli and show similarities to and differences from those of theirA. tumefaciens counterparts, contributing to an understanding and a comparison of these two systems. Whereas mostvir genes inR. etli follow an induction pattern similar to that ofA. tumefaciens vir genes, a few significant differences may at least in part explain the variations in T-DNA transfer efficiency.