INSERTION SEQUENCES

Insertion sequences (ISs) constitute an important component of most ba cterial genomes. Over 500 individual ISs have been described in the li terature to dare, and marry more are being discovered in the ongoing p rokaryotic and eukaryotic genome-sequencing projects. The last 10 year s have also seen some striking advances in our understanding of the tr ansposition process itself: Not least of these has been the developmen t of various in vitro transposition systems for both prokaryotic and e ukaryotic elements and for several of these, a detailed understanding of the transposition process at the chemical level. This review presen ts a general overview of the organization and function of insertion se quences of eubacterial, archaebacterial, and eukaryotic origins with p articular emphasis on bacterial elements and on different aspects of t he transposition mechanism. It also attempts to provide a framework fo r classification of these elements by assigning them to various famili es or groups. A total of 443 members of the collection have been group ed in 17 families bused on combinations of the following criteria: (i) similarities in generic organization (arrangement of open reading fra mes); (ii) marked identities or similarities in the enzymes which medi ate the transposition reactions, the recombinases/ transposases (Tpase s); (iii) similar features of their ends (terminal IRs); and (iv) fate of the nucleotide sequence of their target sites (generation of a dir ect target duplication of determined length). A brief description of t he mechanism(s) involved in the mobility of individual ISs in each fam ily and of the structure;function relationships of the individual Tpas es is included where available.