Factors affecting transposition activity of IS50 and Tn5 ends

The partially matched I and 0 ends of IS50 (the insertion sequence of the transposon Tn5) are needed for transposition, probably as the sites upon which the cis-acting transposase and host proteins act. To better understand how transposition is regulated we made a series of IS50-related elements in which the positions of the ends and of the transposase gene were varied systematically. Assays of these elements showed that the I and 0 ends differ inherently in transposition activity. Other workers showed that methylation, at DNA N⁶-adenine methyltransferase (Dam) recognition sites within the I end and the transposase tnp gene promoter, inhibits transposase synthesis and also I end activity. We show that the effect of Dammediated methylation on an I end depends on the end's orientation relative to the tnp gene. Further, in dam⁺ cells the relative activity of the four possible pairs of ends (when the tnp gene is oriented like -tnp-→ in relation to the first and second ends) are (O, I)>(O, O)≥(I, O)>(I, I). In dam^- cells the relative activities are (O, I)=(I, O)=(I, I)>(O, O). Our results are consistent with a model originally developed for IS70, in which hemi-methylation resulting from passage of a replication fork regulates transposition.