Escherichia coli Insertion Sequence IS 150 : Transposition via Circular and Linear Intermediates

Abstract

ABSTRACT IS 150 , a member of the widespread IS 3 family, contains two consecutive out-of-phase open reading frames, orfA and orfB , that partially overlap. These open reading frames encode three proteins, InsA, InsB, and the InsAB protein, which is jointly encoded by both open reading frames by means of programmed translational frameshifting. We demonstrate that the InsAB protein represents the IS 150 element's transposase. In vivo, the wild-type IS 150 element generates circular excision products and linear IS 150 molecules. Circular and linear species have previously been detected with mutant derivatives of other members of the IS 3 family. Our finding supports the assumption that these products represent true transposition intermediates of members of this family. Analysis of the molecular nature of these two species suggested that the circular forms are precursors of the linear molecules. Elimination of InsA synthesis within the otherwise intact element led to accumulation of large amounts of the linear species, indicating that the primary role of InsA may be to prevent abortive production of the linear species and to couple generation of these species to productive insertion events.