DNA sequence at the end of IS1 required for transposition

Abstract

The insertion sequence IS1 belongs to a class of bacterial transposable genetic elements that can form compound transposons in which two copies of IS1 flank an otherwise non-transposable segment of DNA. IS1 differs from other known elements of this class (such as IS 10, IS 50 and IS 903) in several respects. It is one of the smallest known insertion elements^1–3, exhibits a relatively complex array of open reading frames³, is present in the chromosomes of various Enterobacteria, in some cases in many copies^4,5, and its insertion can result in the duplication of either 8 or 9 base pairs (bp) in the target DNA³. Furthermore, although, like other members of the compound class, it seems to undergo direct transposition, IS1 also promotes replicon fusion (co-integrate formation) at a relatively high frequency⁶. Like all other elements studied to date, the integrity of the extremities of IS1 are essential for efficient transposition⁷. We have constructed a test system to determine the minimal DNA sequences at the extremities of IS1 required for transposition. Sequential deletions of the end sequences reveal that 21–25 bp of an isolated extremity are sufficient for transposition. A specific sequence 13–23 bp from the ends, defining the edge of the minimal sequence, is implicated as an essential site. The sites, symmetrically arrayed at both ends of IS1, correspond to the apparent consensus sequence of the known binding sites for the Escherichia coli DNA-binding protein (called integration host factor or IHF) which is required for the site-specific recombination that leads to integration of bacteriophage λ into the bacterial genome^8,9,26. The sites at the ends of IS1 may thus bind a host protein, such as JHF or a related protein, that is involved in regulating the transposition apparatus.