Transient promoter formation: A new feedback mechanism for regulation of IS911 transposition

Abstract

IS911 transposition involves a free circular transposon intermediate where the terminal inverted repeat sequences are connected. Transposase synthesis is usually driven by a weak promoter, PIRL, in the left end (IRL). Circle junction formation creates a strong promoter, Pjunc, with a -35 sequence located in the right end and the -10 sequence in the left. Pjunc assembly would permit an increase in synthesis of transposase from the transposon circle, which would be expected to stimulate integration. Insertion results in Pjunc disassembly and a return to the low PIRL-driven transposase levels. We demonstrate that Pjunc plays an important role in regulating IS911 transposition. Inactivation of Pjunc strongly decreased IS911 transposition when transposase was produced in its natural configuration. This novel feedback mechanism permits transient and controlled activation of integration only in the presence of the correct (circular) intermediate. We have also investigated other members of the IS3 and other IS families. Several, but not all, IS3 family members possess Pjunc equivalents, underlining that the regulatory mechanisms adopted to fine-tune transposition may be different.