Two pathways in bacteriophage Mu transposition

The bacteriophage Mu can insert its DNA at a large number of sites in the Escherichia coli chromosome and mediates a variety of chromosomal rearrangements (Taylor 1963; Bukhari and Zipser 1972; Danietl et al. 1972; Faelen and Toussaint 1978). Since Mu DNA has never been found in its free form but always appears to be associated with bacterial DNA, the integration of Mu resembles the transposition of insertion elements (for review, see Bukhari 1976). However, the only direct proof that the mechanisms of transposition of Mu and insertion elements are related stems from the observa-tion that upon integration of Mu, as upon transposition of insertion elements, a segment of host DNA is dupli-cated at the target site (for review, see Calos and Miller 1980). The main difference between Mu and insertion elements lies in the efficiency of the respective transposi-tion systems. Furthermore, transposition of Mu in lyric development seems to be the main cause for the rapid amplification of Mu DNA. The linkage between trans-position and replication is illustrated by the fact that the phage attachment sites are also required for replication, and the products of genes A and B, which are required for replication, are also involved in transposition (O'Day et al. 1978; van de Putte t al. 1978). However, the A-gene product is essential for integration, whereas the B-gene product has only a stimulatory effect (O'Day et al. 1978). Both precise and imprecise xcision of Mu require the A-gene product and the absence of the B-gene product (Bukhari 1975). Our studies have focused on the structural features of bacteriophage Mu integration, and we shall discuss pos-sible implications of these results with respect to the ex-istence of different pathways for transposition