Replicative Transposition Proceeds Through a Cointegrate

KEY CONCEPTS
- Replication of a strand transfer complex generates a cointegrate, which is a fusion of the donor and target replicons.
- The cointegrate has two copies of the transposon, which lie between the original replicons.
- Recombination between the transposon copies regenerates the original replicons, but the recipient has gained a copy of the transposon.
- The recombination reaction is catalyzed by a resolvase coded by the transposon.
The basic structures involved in replicative transposition are illustrated in FIGURE 1: The 3′ ends of the strand transfer complex are used as primers for replication. This generates a structure called a cointegrate, which represents a fusion of the two original molecules. The cointegrate has two copies of the transposon, one at each junction between the original replicons, oriented as direct repeats. The crossover is formed by the transposase. Its conversion into the cointegrate requires host replication functions.

FIGURE 1.Transposition may fuse a donor and recipient replicon into a cointegrate. Resolution releases two replicons, each containing a copy of the transposon.
Homologous recombination between the two copies of the transposon releases two individual replicons, each of which has a copy of the transposon. One of the replicons is the original donor replicon. The other is a target replicon that has gained a transposon flanked by short direct repeats of the host target sequence. The recombination reaction is called resolution; the enzyme activity responsible is called the resolvase.
The reactions involved in generating a cointegrate have been defined in detail for phage Mu and are illustrated in FIGURE 2. The process starts with the formation of the strand transfer complex (sometimes called a crossover complex). The donor and target strands are ligated so that each end of the transposon sequence is joined to one of the protruding single strands generated at the target site. The strand transfer complex generates a crossover-shaped structure held together at the duplex transposon. The fate of the crossover structure determines the mode of transposition.

FIGURE 2. Mu transposition generates a crossover structure, which is converted by replication into a cointegrate.
The principle of replicative transposition is that replication through the transposon duplicates it, which creates copies at both the target and donor sites. The product is a cointegrate.
The crossover structure contains a single-stranded region at each of the staggered ends. These regions are pseudoreplication forks that provide a template for DNA synthesis. (Use of the ends as primers for replication implies that the strand breakage must occur with a polarity that generates a 3′–OH terminus at this point.)
If replication continues from both of the pseudoreplication forks, it will proceed through the transposon, separating its strands and terminating at its ends. Replication is accomplished by hostencoded functions. At this juncture, the structure has become a cointegrate, possessing direct repeats of the transposon at the junctions between the replicons (as can be seen by tracing the path around the cointegrate).