Natural Instability of Agrobacterium vitis Ti Plasmid Due to Unusual Duplication of a 2.3-kb DNA Fragment. Pascal Fournier. Plant Pathology Department, C.N.R.S. Instute of Plant Molecular Biology, Rue du General Zimmer 12, Strasbourg 67084, France. Patrice de Rufray, Leon Otten. Plant Pathology Department, C.N.R.S. Instute of Plant Molecular Biology, Rue du General Zimmer 12, Strasbourg 67084, France. MPMI 7:164-172. Accepted 15 November 1993. This article in in the publc domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994.

The octopine/cucumpopine (o/c) Ti plasmids of Agrobacterium vitis carry two T regions, TA and TB. The TA region resembles the octopine TL region. The TB region contains the auxin synthesis genes TB-iaaM and TB-iaaH and the cucumopine synthesis gene cus. Within the group of o/c isolates, strains 2608 and 2641 are closely related. However, 2641 lacks the TB region. The restriction maps of pTi2608 and pTi2641 were established and showed that the TB deletion resulted from intramolecular recombination between two directly repeated sequences separated by 66 kb in pTi2608. the 2,294-bp repeated sequences lacks inverted repeats and does not duplicate its target site, indicating that it is not a classical bacterial insertion sequence (IS element). It was therefore called an RSAv element (repeated sequence of A. vitis). The RSAv element carries two open reading frames: ORF234 is homologous to the traR gene of the A. tumefaciens nopaline Ti Plasmid pTiC58; ORF488 is homologous to the sucrose phosphorylase gene of Leuconostoc mesenteroides and the glucosyl transferase A gene of Streptococcus mutans. The RSAv repeat starts precisely at the start codon of ORF488 and ends two base pairs 3' of the stop codon of ORF234. The structural organization of the RSAv element suggests that the amplification agent did not result from a random amplification process. a study of the distribution of the two RSAv copies(RSAv-1 and RSAv-2) in pTi2608 and other o/c isolates indicates that the ancestor o/c Ti plasmid contained only RSAv-1 and that this sequence was duplicated at one point during the divergent evolution of the o/c Ti plasmids. Repeated sub culturing of strain 2608 resulted in the same deletion in the Ti plasmid as that found in strain 2641, demonstrating that the Ti plasmids of o/c A. visit strains wiht two RSAv copies are unstable.