HORIZONTAL GENE-TRANSFER - REGULATED EXPRESSION OF A TOBACCO HOMOLOG OF THE AGROBACTERIUM-RHIZOGENES ROIC GENE

A tobacco homologue (trolC) of the rolC gene of the Agrobacterium rhiz ogenes Ri-plasmid was cloned and sequenced from Nicotiana tabacum L. c v. Havana 425. The coding region of trolC is similar in sequence (69-8 7% for DNA and 54-89% for the deduced amino acid sequence) to rolC gen es of the agropine, mannopine, and mikimopine strains of Ri-plasmids a nd the N. glauca rolC homologue. Southern analyses showed that trolC i s encoded by a small gene family derived from the tomentosiformis ance stor of tobacco. This suggests that trolC resulted from an ancient tra nsfer 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. Accumulatio n of trolC mRNA in cultured leaf tissues was strongly down-regulated b y auxin and induced by cytokinin. These results are of particular inte rest because they suggest that a gene of bacterial origin introduced d uring evolution can have a function in a modern plant.