Since the publication in 1989 of the volume Mobile DNA (29), the field of transposition has experienced some striking advances. Not the least of these has been the development of various in vitro transposition systems for both prokaryotic and eukaryotic elements and an understanding of the transposition process at the chemical level for several of these (for reviews, see references 66, 179, 239, and 266). Another development has been the veritable explosion in the number of different transposable elements isolated and characterized at the nucleotide sequence level. In the case of bacterial insertion sequences (IS), only approximately 50 had been analysed at this level in 1989 (107), compared to over 500 today. This is equally true of eukaryotic "insertion sequences" such as mariner, derivatives of which have been found in over 240 insect species in addition to fungi, mammals, fish, and plants (295), and related elements such as Tc1 (87, 274). This enormous diversity and distribution is astonishing. The number of different ISs has now become so great that there is a growing need for a framework to enable a systematic classification. We have attempted here to provide a classification of these elements by assigning them to various families or groups based on similarities and differences in structure, organization, and nucleotide and protein sequence relationships. We also include a brief description, where available, of the mechanism(s) involved in the mobility of individual ISs and of the structure-function relationships of the enzymes which mediate the transposition reactions, the recombinases/transposases (Tpases). Most of the elements of this framework are already available but scattered throughout the literature.
In assembling this review, we have limited our treatment to IS elements, which we loosely define as small (<2.5-kb), phenotypically cryptic segments of DNA with a simple genetic organization and capable of inserting at multiple sites in a target molecule. In this definition, we voluntarily eliminate several types of mobile genetic element. These include elements with RNA as intermediates such as the retroviruses, retrotransposons and retroposons (102, 315), DNA elements such as the conjugative transposons (306) which use a phage 
-type mechanism for their translocation, and elements such as bacteriophage Mu (239), Tn7 (66), and transposons of the Tn554 type (249), which are large and relatively complex. We have also eliminated an additional large and coherent group of elements, the type II transposons of the Tn3 family (373). Although several are small and might qualify as insertion sequences [e.g., Tn1000 (
) and IS101 (324) and IS1071 (86a, 251)], many are complicated in structure and include multiple antibiotic resistance genes carried by another type of transposable element, the integron (278).
Although they are not considered here in detail, many of these elements have significant functional similarities to ISs and have provided many of the important insights into transposition mechanism. Reference to these is included where they may prove useful to understanding the behavior of IS elements.
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