Replicative and conservative transposition in bacteria

Abstract

When a bacterial transposon jumps to a new genomic location, does it replicate itself in the process thus generating two transposon copies, or does it excise itself from the old site and integrate into the new site without duplication? eplicative or conservative: this has been a central question concerning the mechanism of transposition. The lack of any well-characterized in vitro systems for studying transposition (with the exception of the bacteriophage Mu system) has restricted progress. However, thanks to some elegant genetic experiments, the answer now seems much clearer. Replicative Transposition Most mechanistic studies have been performed with members of three common groups of bacterial transposons: (i) the insertion sequences (IS) and their composite transposons; (ii) the Tn3 family of transposons; and (iii) the transposing bacteriophage Mu. The case for a replicative mechanism of transposition was established early and was a strong one. It came on two fronts-from studies of Tn3 and bacteriophage Mu. Transposition of Tn3 and related elements generally occurs by a two-step process (see Grindley, Cell 32, 3-5, 1983; Heffron, in Mobile Genetic Elements, ed. J. Shapiro, Academic Press, 1983, pp. 223-260). First, the donor and target replicons are fused to form a cointegrate molecule, which has a copy of the transposon at each junction of target and donor DNA (Figure 1). A site-specific recombination between the two transposon copies then reduces the cointegrate into its two constituent replicons, regenerating the donor and forming the target with a simple insertion. The two steps involve the consecutive action of two proteins, transposase and resolvase, both encoded by the transposon. The replicative nature of the transposasedependent step is clear from the nature of the cointegrate intermediate; although formed from a target and a donor with a single transposon copy, it contains two copies of the transposon. The replicative nature of Mu transposition was evident from the fact that replication of the phage during a lytic infection was absolutely dependent upon its transposition to many sites in the genome of a single cell. The short segments of random bacterial DNA joined to the ends of Mu in viral particles are a signature of this orgy of transposition. Elegant biochemical studies have firmly established that Mu transposition is generally replicative and, in addition, have elucidated the structure of an intermediate (see below) formed through the action of the Mu transposase (A protein), the B protein, and the E. coli histone-like protein HU (Mizuuchi, Cell 35, 785-794, 1983; Craigie and ireview