LOCALIZATION OF ACTION OF THE IS50-ENCODED TRANSPOSASE PROTEIN

Suhas H. Phadnis ¹, Chihiro Sasakawa ², and Douglas E. Berg ³

¹ Department of Microbiology, Box 8093, Washington University School of Medicine, St. Louis, Missouri 63110
² Department of Immunology, Box 8093, Washington University School of Medicine, St. Louis, Missouri 63110
³ Department of Genetics, Box 8093, Washington University School of Medicine, St. Louis, Missouri 63110

The movement of the bacterial insertion sequence IS50 and of composite elements containing direct terminal repeats of IS50 involves the two ends of IS50, designated O (outside) and I (inside), which are weakly matched in DNA sequence, and an IS50 encoded protein, transposase, which recognizes the O and I ends and acts preferentially in cis. Previous data had suggested that, initially, transposase interacts preferentially with the O end sequence and then, in a second step, with either an O or an I end. To better understand the cis action of transposase and how IS50 ends are selected, we generated a series of composite transposons which contain direct repeats of IS50 elements. In each transposon, one IS50 element encoded transposase (tnp ⁺), and the other contained a null (tnp^-) allele. In each of the five sets of composite transposons studied, the transposon for which the tnp⁺ IS50 element contained its O end was more active than a complementary transposon for which the tnp ^- IS50 element contained its O end. This pattern of O end use suggests models in which the cis action of transposase and its choice of ends is determined by protein tracking along DNA molecules.

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