Characterization of a putative RND-type efflux system in Agrobacterium tumefaciens

Sequencing of a 7277 bp fragment adjacent to the chvH locus of Agrobacterium tumefaciens revealed four open reading frames (ORFs), designated ameR, ameA, ameB and ameC, respectively. These ORFs exhibit amino acid similarities to components of Resistance-Nodulation-Cell Division (RND) type efflux systems. AmeA and AmeB show high homology to membrane fusion proteins (MFP) and RND-type transporters, whereas AmeC shows similarity to NodT and other members of outer membrane factor families. Mutations of the ameA and ameB genes did not affect the susceptibility profile of the wild-type strain to several detergents and antibiotics. In contrast, mutations of the ameC gene dramatically affected the susceptibility of the strain to these same inhibitory compounds. RT-PCR analysis demonstrated that the ameABC genes form an operon. In addition, ameC gene has its own promoter gene located in the intergenic region between ameB and ameC. Mapping upstream of ameA is ameR, which encodes a protein that shows similarity especially at its N-terminal end to the TetR family of bacterial transcriptional regulators. AmeR negatively regulates expression of the ameABC operon. A mutation in ameR increased the resistance of the cells to several antimicrobial agents. This regulatory locus appears to be in the same operon as a gene located upstream which codes for a product that has high similarity to numerous 4-(N-succinocarboxamide)-5-aminoimidazole ribonucleotide (SAICAR) synthetases. The possible role of the putative efflux pump coded by the ame genes is discussed.