ASSEMBLY OF A STRONG PROMOTER FOLLOWING IS911 CIRCULARIZATION AND THEROLE OF CIRCLES IN TRANSPOSITION

When supplied with high levels of the IS911-encoded transposase, IS911 -based transposons can excise as circles in which the right and left t erminal inverted repeats are abutted. Formation of the circle junction is shown here to create a promoter, p(junc), which is significantly s tronger than the indigenous promoter, p(IRL), and is also capable of d riving expression of the IS911 transposition proteins. High transposas e expression from the circular transposon may promote use of the circl e as an integration substrate. The results demonstrate that IS911 circ les are highly efficient substrates for insertion into a target molecu le in vivo, Insertion leads to the disassembly of p(junc) and thus to a lower level of synthesis of the transposition proteins. The observat ion that normal levels of IS911 transposition proteins supplied by wil d-type copies of IS911 are also capable of generating transposon circl es, albeit at a low level, reinforces the idea that the transposon cir cles might form part of the natural transposition cycle of IS911. Thes e observations form the elements of a feedback control mechanism and h ave been incorporated into a model describing one possible pathway of IS911 transposition.