Polynucleotidyl transfer reactions in transpositional DNA recombination.

The transposon family of mobile genetic elements is widespread among organisms. While only a handful of these elements have been analyzed biochemically, all the elements that have been studied in detail share similar reaction steps. These steps that compose the transpositional recombination process are two distinct types of polynucleotidyl transfer reactions which generate a strand transfer product, a critical transposition intermediate (Fig. 1). In the first chemical step of transpositional recombination, a pair of site-specific endonucleolytic cleavages separates the 3‘-OH of the transposon DNA ends from the 5’-phosphoryl ends of the adjoining host DNA. In other words, the polynucleotide 5’-phosphate that was connected to the 3’-ends of the transposon sequence is transferred to a water molecule. While in Mu transposition (Fig. lA) and in retroviral DNA integration (Fig. lB), only one strand at each end of the element sequence is cut, in the cases of TnlO or Tn7, the opposite strand is also cut at (TnlO) or near (Tn7) the end (Fig. 1C). In the second step, two strands of a target DNA undergo a transfer reaction that disconnects the 5’-phosphoryl from its original 3”partner and joins it to the 3’-OH ends of the donor transposon sequence. The resulting transposition intermediate is processed by the DNA damage repair or DNA replication machinery of the host organism to generate final products. While the type of intermediate produced by Mu (Fig. lA ) can be processed by replication of the element DNA sequence, those made by TnlO (Fig. 1C) or retroelements (Fig. 1B) can be processed only by gap repair. Recent findings on the mechanism of the two chemical steps of transpositional recombination are the subject of this review. A part of the subject considered here has also been reviewed in Ref. 1 and in more depth in Ref. 2. Information on a wider variety of transposable elements is found in Ref. 3.