Specificity of insertion of IS1.

A systematic study of the specificity of insertion of the transposable element IS1 into small defined-sequence plasmids (pBR322 and derivatives) was conducted to determine the features of the DNA sequence that influence target site selection. We have physically mapped several collections of independent insertions of IS1 into these plasmids and have determined: (1) that about 80% of all insertions occur in the DNA segment (about 200 base-pairs) between the unique EcoRI site of pBR322 and the beginning of the beta-lactamase gene, one of the two regions of high A + T density in this plasmid; (2) that there is a strong orientation effect in this region (almost all IS1 insertions are in one orientation) that depends on both the pBR322 sequence and the environment of the transposon in the donor molecule; and (3) that the orientation effect does not depend on the strong transcription that is directed through this region in pBR322. Furthermore, we have found that insertion of a poly(dA X dT) segment into pBR322 creates an artificial hotspot for IS1 insertion, even though it is not as attractive for insertion as the above-mentioned major hotspot. Our observations suggest that an interplay between several properties of the target sequences and the sequence environment of the donor transposon is responsible for the observed specificity of position and orientation. One of the possibilities discussed here is that preferred "entry-sites", or "signal" sequences, for the transposition complex play a major role in determining the positions and orientations of IS1 insertions.