Target Choice and Orientation Preference of the Insertion Sequence IS903

We have examined the targeting preference of the bacterial insertion element IS903 by determining the sites of insertion of a large number of transposition events into the 55-kb conjugative plasmid pOX38. Despite the large target size, all the insertions were clustered in four small distinct regions associated with conjugal DNA transfer. Within these regions, many different sites were used for insertion; however, there were a few sites that IS903 inserted into more than once. Alignment of the insertion sites showed that there was no consensus sequence within the 9-bp target duplication but that there were preferred sequences located symmetrically on either side of the target. This is consistent with target recognition by a dimer or multimer of transposase, with either sequence-specific or structure-specific interactions on both sides of the target. We show further that when one of these preferred regions was cloned into a second conjugative plasmid, pUB307, it was still a preferred target, implying that all the sequences necessary for target selection are contained within this DNA segment. Also, we observed a very strong preference for insertion in a single orientation in pUB307. We examined the possibility that either DNA replication from the origin of vegetative replication, oriV, or the origin of transfer, oriT, might determine this orientation effect. We find that reversing the direction of vegetative replication had no effect on the orientation of transposon insertions; however, reversing the direction of DNA transfer abolished the orientation effect. This supports the idea that conjugal DNA transfer imparts a polarity on the target that is sensed by the transposon.