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Some Bacterial Transposable Elements: Their Organization, Mechanisms of Transposition, and Roles in Genome Evolution

William S. Reznikoff


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)...

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