AGROBACTERIUM VITIS NOPALINE TI PLASMID PTIAB4 - RELATIONSHIP TO OTHER TI PLASMIDS AND T-DNA STRUCTURE

The Ti plasmid of the Agrobacterium vitis nopaline-type strain AB4 was subcloned and mapped. Several regions of the 157 kb Ti plasmid are si milar or identical to parts of the A. vitis octopine/cucumopine (o/c)- type Ti plasmids, and other regions are homologous to the nopaline-typ e Ti plasmid pTiC58. The T-DNA of pTiAB4 is a chimaeric structure of r ecent origin: the left part is 99.2% homologous to the left part of th e TA-DNA of the o/c-type Ti plasmids, while the right part is 97.1% ho mologous to the right part of an unusual nopaline T-DNA recently ident ified in strain 82.139, a biotype II strain from wild cherry. The 3' n oncoding regions of the ist genes from pTiAB4 and pTi82.139 are differ ent from those of other ipt genes and contain a 62 bp fragment derived from the coding sequence of an ipt gene of unknown origin. A comparis on of different ipt gene sequences indicates that the corresponding 62 bp sequence within the coding region of the AB4 ipt gene has been mod ified during the course of its evolution, apparently by sequence trans fer from the 62bp sequence in the 3' non-coding region. In pTi82.139 t he original coding region of the ipt gene has remained largely unmodif ied. The pTiAB4 6b gene differs from its pTi82.139 counterpart by the lack of a 12 bp repeat in the 3' part of the coding sequence. This lea ds to the loss of four glutamic acid residues from a series of ten. In spite of these differences, the ipt and 6b genes of pTiAB4 are functi onal. Our results provide new insight into the evolution of Agrobacter ium Ti plasmids and confirm the remarkable plasticity of these genetic elements. Possible implications for the study of bacterial phylogeny are discussed.