Rapid identification and mapping of insertion sequences in Escherichia coli genomes using vectorette PCR

Insertion sequences (IS) are small DNA segments capable of transposing within and between prokaryotic genomes, often causing insertional mutations and chromosomal rearrangements. Although several methods are available for locating ISs in microbial genomes, they are either labor-intensive or inefficient. Here, we use vectorette PCR to identify and map the genomic positions of the eight insertion sequences (IS1, 2, 3, 4, 5, 30, 150, and 186) found in E. coli strain CGSC6300, a close relative of MG1655 whose genome has been sequenced.

Genomic DNA from strain CGSC6300 was digested with a four-base cutter Rsa I and the resulting restriction fragments ligated onto vectorette units. Using IS-specific primers directed outward from the extreme ends of each IS and a vectorette primer, flanking DNA fragments were amplified from all but one of the 37 IS elements identified in the genomic sequence of MG1655. Purification and sequencing of the PCR products confirmed that they are IS-associated flanking DNA fragments corresponding to the known IS locations in the MG1655 genome. Seven additional insertions were found in strain CGSC6300 indicating that very closely related isolates of the same laboratory strain (the K12 isolate) may differ in their IS complement. Two other E. coli K12 derivatives, TD2 and TD10, were also analyzed by vectorette PCR. They share 36 of the MG1655 IS sites as well as having 16 and 18 additional insertions, respectively.

This study shows that vectorette PCR is a swift, efficient, reliable method for typing microbial strains and identifying and mapping IS insertion sites present in microbial genomes. Unlike Southern hybridization and inverse PCR, our approach involves only one genomic digest and one ligation step. Vectorette PCR is then used to simultaneously amplify all IS elements of a given type, making it a rapid and sensitive means to survey IS elements in genomes. The ability to rapidly identify the IS complements of microbial genomes should facilitate subtyping closely related pathogens during disease outbreaks.