General features and properties of insertion sequence elements

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Programmed translational frameshifting

A second mechanism acts at the level of translation elongation and involves programmed translational frameshifting between two consecutive open reading frames (Fig 1.33.1). Typically a -1 frameshift is observed in which the translating ribosome slides one base upstream and resumes in the alternative phase. This generally occurs at the position of so-called slippery codons in a heptanucleotide sequence of the type X XXZ ZZN in phase 0 (where the bases paired with the anticodon are shown as triplets) which is read as XXX ZZZ N in the shifted -1 phase (Fig 1.33.1) (see e.g. (Chandler & Fayet, 1993), (Farabaugh, 1996, Farabaugh, 1997), (Gesteland & Atkins, 1996), The sequence A AAA AAG is a common example of this type of heptanucleotide. Ribosomal shifting of this type is stimulated by structures in the mRNA which tend to impede the progression of the ribosome such as potential ribosome binding sites upstream or secondary structures (stem-loop structures and pseudoknots) downstream of the slippery codons (Farabaugh, 1997). Translational control of transposition by frameshifting has been demonstrated both for IS1 (Sekine & Ohtsubo, 1989`) (Escoubas, et al., 1991), and for members of the IS3 family (Fig 1.33.2) ((Polard, et al., 1991); see also (Chandler & Fayet, 1993) (Fayet & Prère, 2010)) but may also occur in several other IS elements (see for example IS5 and IS630 families). For IS1 and members of the IS3 family, the upstream frame appears to carry a DNA recognition domain whereas the downstream frame encodes the catalytic site. While the product of the upstream frame alone acts as a modulator of activity, presumably by binding to the IR sequences, frameshifting assembles both domains into a single protein, the Tpase, which directs the cleavages and strand transfer necessary for mobility of the element. The frameshifting frequency is thus critical in determining overall transposition activity. Although it has yet to be explored in detail, frameshifting could be influenced by host physiology thus coupling transposition activity to the state of the host cell.

    References :
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