Rapid mapping of transposon insertion and deletion mutations in the large Ti-plasmids of Agrobacterium tumefaciens

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Abstract
A procedure is presented, that has allowed the rapid assignment of transposon Tn1 and Tn7 insertion sites in the large (130 Md) nopaline Ti-plasmid pTiC58, to specific restriction enzyme fragments. Total bacterial DNA is isolated from Agrobacterium tumefaciens strain C58 mutants that carry a transposon in their Ti-plasmid, and digested with an appropriate restriction endonuclease. The fragments are separated on an agarose gel, denatured and transferred to nitrocellulose filters. These are hybridized against purified wild type pTiC58, or against segments of PTiC58, cloned in E. coli using pBR322 as a vector plasmid. DNA sequences homologous to the probe are detected by autoradiography, thus generating a restriction enzyme pattern of the plasmid from a digest of total bacterial DNA. Mutant fragments can be readily identified by their different position compared to a wild type reference. This protocol eliminates the need to separate the large plasmid from chromosomal DNA for every mutant. In principle, it can be applied to the restriction enzyme analysis of insertion or deletion mutants in any plasmid that has no extensive homology with the chromosome.