Replicative transposition

Abstract: The lytic cycle of bacteriophage Mu includes a large number of coupled DNA replication and integration events, each of which is equivalent in several respects to the process of transposition of genetic elements. To aid us in studying the process of Mu DNA replicative transposition, we developed a technique for synchronizing the first round of replication following induction of a lysogen. Synchronization was achieved by inducing a lysogen in the absence of DNA replication for a time sufficient to develop the potential for Mu DNA replication in all cells in the population; upon release of the inhibition of replication, a synchronized round of Mu DNA replication was observed. Development of the potential for Mu DNA replication in the entire population took approximately 12 min. Protein synthesis was required for development of the potential, but the requirement for protein synthesis was satisfied by approximately 9 min suggesting that other, as yet unspecified, reactions occupied the last 3 min. Replication proceeded predominantly from the left end of the prophage, though a significant amount of initiation from the right end was observed. The usefulness of the technique for studying the mechanism of replicative transposition and the end products of a single round of replication are discussed.

Abstract: We have comprehensively analyzed the linear chromosomes of Streptomyces griseus mutants constructed and kept in our laboratory. During this study, macrorestriction analysis of AseI and DraI fragments of mutant 402-2 suggested a large chromosomal inversion. The junctions of chromosomal inversion were cloned and sequenced and compared with the corresponding target sequences in the parent strain 2247. Consequently, a transposon-involved mechanism was revealed. Namely, a transposon originally located at the left target site was replicatively transposed to the right target site in an inverted direction, which generated a second copy and at the same time caused a 2.5-Mb chromosomal inversion. The involved transposon named TnSGR was grouped into a new subfamily of the resolvase-encoding Tn3 family transposons based on its gene organization. At the end, terminal diversity of S. griseus chromosomes is discussed by comparing the sequences of strains 2247 and IFO13350.

Abstract: Transposable elements are DNA sequences that encode catalytic functions for and participate in the genetic process termed transposition. This process (which is pictorially described in Fig. 1) involves the translocation of the transposable element from one site to a second site (termed a target site and indicated in Fig. 1). This target site is duplicated during transposition with one copy of it being found on either side of the transposed element. In many cases (perhaps all), the transposition process involves replication and recombination events—a copy of the transposable element remains in its original site, and a copy appears at the new site (a possible exception to this is discussed later). Transposition events generate insertion mutations, which disrupt the integrity of the target DNAs. In addition, since transposable elements can carry transcription initiation and/or termination signals, they can alter downstream gene expression. Transposable elements also catalyze other genetic events, including (1) deletion formation, (2) inversion formation, and (3) replicon fusion (cointegrate formation). The deletion and inversion events can be explained as being products of intrareplicon transposition events (Shapiro 1979). Cointegrates are intermediates in the transposition pathway for some transposable elements, and they may be the product of an alternative pathway for other transposable elements. Cointegrate formation involves the fusion of the replicon carrying a transposable element with a target site on a second replicon, with the concomitant duplication of the transposable element. This process (shown schematically in Fig. 2) has been elucidated by the experiments of Muster and Shapiro (1981)...

Abstract: Phage Mu is the paradigm of a growing family of bacteriophages that infect a wide range of bacterial species and replicate their genome by replicative transposition. This molecular process, which is used by other mobile genetic elements to move within genomes, involves the profound rearrangement of the host genome [chromosome(s) and plasmid(s)] and can be exploited for the genetic analysis of the host bacteria and the in vivo cloning of host genes. In this chapter we review Mu-derived constructs that optimize the phage as a series of genetic tools that could inspire the development of similarly efficient tools from other transposable phages for a large spectrum of bacteria.