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Journal of Bacteriology, February 2004, p. 858-865, Vol. 186, No. 3
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.3.858-865.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

The Left End of IS2: a Compromise between Transpositional Activity and an Essential Promoter Function That Regulates the Transposition Pathway

Leslie A. Lewis,^1,2* Edruge Cylin,¹ Ho Kyung Lee,¹ Robert Saby,¹ Wilson Wong,^1, and Nigel D. F. Grindley³

Department of Biology, York College of the City University of New York, Jamaica, New York 11451,¹ Program in Cellular, Molecular and Developmental Biology, Graduate Center, City University of New York, New York, New York 11016,² Department of Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520³

Received 23 June 2003/ Accepted 20 October 2003

Cut-and-paste (simple insertion) and replicative transposition pathways are the two classical paradigms by which transposable elements are mobilized. A novel variation of cut and paste, a two-step transposition cycle, has recently been proposed for insertion sequences of the IS3 family. In IS2 this variation involves the formation of a circular, putative transposition intermediate (the minicircle) in the first step. Two aspects of the minicircle may involve its proposed role in the second step (integration into the target). The first is the presence of a highly reactive junction formed by the two abutted ends of the element. The second is the assembly at the minicircle junction of a strong hybrid promoter which generates higher levels of transposase. In this report we show that IS2 possesses a highly reactive minicircle junction at which a strong promoter is assembled and that the promoter is needed for the efficient completion of the pathway. We show that the sequence diversions which characterize the imperfect inverted repeats or ends of this element have evolved specifically to permit the formation and optimal function of this promoter. While these sequence diversions eliminate catalytic activity of the left end (IRL) in the linear element, sufficient sequence information essential for catalysis is retained by the IRL in the context of the minicircle junction. These data confirm that the minicircle is an essential intermediate in the two-step transposition pathway of IS2.

Present address: Graduate Program in Biology, Queens College, City University of New York, Flushing, NY 11365.

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