Agrobacterium transcriptional regulator Ros is a prokaryotic zinc finger protein that regulates the plant oncogene ipt

Virulence genes of Agrobacterium tumefaciens are under the control of positive and negative transcriptional regulators. We found that the transcriptional regulator Ros controls expression of the plant oncogene ipt, which encodes isopentenyl transferase, in A. tumefaciens. This enzyme is involved in biosynthesis of the plant growth hormone cytokinin in the host plant. An ipt promoter::cat reporter gene fusion showed a 10-fold increase in ipt promoter activity in A. tumefaciens ros mutant strains when compared with wild type. Also, increased levels (10- to 20-fold) of isopentenyl adenosine, the product of the reaction catalyzed by isopentenyl transferase, were detected in ros mutant strains. In vitro studies using purified Ros showed it binds directly to the ipt promoter. Analysis of the deduced Ros amino acid sequence identified a novel type of C2H2 zinc finger. In Ros the peptide loop spacing of the zinc finger is 9 amino acids as opposed to the invariant 12 amino acids in the classical C2H2 motif. Site-directed mutagenesis of Cys-82 and His-92 in this motif showed that these residues are essential for Zn2+ and DNA binding activities of Ros. The existence of such a regulator in Agrobacterium may be due to horizontal interkingdom retrotransfer of the ros gene from plant to bacteria.