Horizontal gene transfer: regulated expression of a tobacco homologue of the Agrobacterium rhizogenes rolC gene.

A tobacco homologue (trolC) of the rolC gene of the Agrobacterium rhizogenes Ri-plasmid was cloned and sequenced from Nicotiana tabacum L. cv. Havana 425. The coding region of trolC is similar in sequence (69-87% for DNA and 54-89% for the deduced amino acid sequence) to rolC genes of the agropine, mannopine, and mikimopine strains of Ri-plasmids and the N. glauca rolC homologue. Southern analyses showed that trolC is encoded by a small gene family derived from the tomentosiformis ancestor of tobacco. This suggests that trolC resulted from an ancient transfer of DNA between A. rhizogenes and a progenitor of modern tobacco. Transcripts of trolC were detected in three morphologically distinct cultivars of tobacco. trolC mRNA accumulated in young leaves and shoot tips, but not in lower leaves and roots of mature plants. Accumulation of trolC mRNA in cultured leaf tissues was strongly down-regulated by auxin and induced by cytokinin. These results are of particular interest because they suggest that a gene of bacterial origin introduced during evolution can have a function in a modern plant.