Minimal region necessary for autonomous replication of pTAR.

The native 44-kilobase-pair plasmid pTAR, discovered in a grapevine strain of Agrobacterium tumefaciens, contains a single origin of DNA replication confined to a 1.0-kilobase-pair region of the macromolecule. This region (ori) confers functions sufficient for replication in Agrobacterium and Rhizobium species but not in Pseudomonas solanacearum, Pseudomonas glumae, Pseudomonas syringae pv. savastanoi, Xanthomonas campestris pv. campestris, and Escherichia coli. ori contains a repA gene that encodes a 28,000-dalton protein required for replication. Nucleotide sequencing of repA and its promoter region revealed four 8-base-pair palindromic repeats upstream of the repA coding region. Deletion of these repeats alters repA expression and plasmid copy number. Downstream of repA are three additional repeats in a region essential for replication. A locus responsible for plasmid partitioning (parA) and a putative second locus regulating plasmid copy number are part of the origin region and are required for stable plasmid maintenance.