With several notable exceptions (the IS91, IS110 and IS200/IS605 families; Table 1) the majority of ISs exhibit
short terminal IR of between 10 and 40 bp. In those cases examined
experimentally, the IRs can be divided into two functional domains (Fig 1.26.1). Domain "b" includes the two or three terminal
base pairs (Fig 1.26.1), and is involved in the cleavages and strand transfer reactions leading
to transposition of the element. Domain "a" is positioned within the IR and is
involved in Tpase binding (Derbyshire & Grindley,
1996), (Derbyshire, et al., 1990), (Huisman, et al.,
1989), (Johnson & Reznikoff, 1983), (Makris,
et al., 1988), (Zerbib, et al., 1990), (Normand, et al.,
2001). A similar organisation has also been proposed for
the transposon Tn3(Ichikawa, et al., 1990) and for the related gd transposon (May & Grindley, 1995). The simple single terminal
Tpase binding sites of ISs are to be contrasted with the multiple and
asymmetric protein binding sites observed in the case of bacteriophage Mu (Craigie, et al., 1984) and
transposons Tn7 (Craig, 1996), and probably
Tn552 (Rowland & Dyke, 1990),(Rowland, et al.,
1995). Multiple protein binding sites are also a characteristic of the complex
En/Spm and Ac elements of maize (see(Gierl, 1996), (Kunze, 1996) (Fig 1.26.2). It is worth noting that members of the IS21 family also carry multiple repeated sequences at both ends which may also represent
Tpase binding sites (Berger, et al., 2001), (Mahillon
& Chandler, 1998).By accommodating
different binding patterns at each end, such an arrangement can provide a
functional distinction between the ends either in the assembly or in the
activity of the synaptic complex. In addition, indigenous IS promoters are
often located partially within the IR sequence upstream of the Tpase gene, by
convention IRL. This arrangement may provide a mechanism for autoregulation of
Tpase synthesis by Tpase binding. Binding sites for host specified proteins are
also often found within or close to the terminal IRs and these proteins may
play a role in modulating transposition activity or Tpase expression.
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