RosR is a transcriptional regulator important for determining cell-surface characteristics and nodulation competitiveness in Rhizobium etli CE3. We identified a 15-kb region that contains genes with similarity to members of the virB, virC, virG, and virE operons of Agrobacterium tumefaciens and demonstrated that RosR directly regulates one operon in this region. These genes were located on plasmid pa of R. etli CE3, which is self-transmissible between R. etli and A. tumefaciens.