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), http://recode.genetics.utah.edu/). 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.
- Chandler M & Fayet O (1993) Translational frameshifting in the control
of transposition in bacteria. Mol
Microbiol 7: 497-503.
- Escoubas JM, Prere MF, Fayet O, Salvignol I, Galas D, Zerbib D &
Chandler M (1991) Translational control of transposition activity of the
bacterial insertion sequence IS1. Embo J 10: 705-712.
- Farabaugh PJ (1996) Programmed translational frameshifting. Microbiol.Rev. 60: 103-134.
- Farabaugh PJ (1997) Programmed
Alternative Reading of the Genetic Code. R.G.Landes Company, Austin.
- Fayet O & Prère M-F (2010) Programmed Ribosomal −1 Frameshifting
as a Tradition: The Bacterial Transposable Elements of the IS3 Family. . Recoding: Expansion of Decoding Rules
Enriches Gene Expression, Vol. 24 (Atkins JF & Gesteland R, eds.),
pp. 259-280. Springer, New York and Heidelberg.
- Gesteland RF & Atkins JF (1996) Recoding: dynamic reprogramming of
translation. Annu.Rev.Biochem. 65: 741-768.
- Polard P, Prère MF, Chandler M & Fayet O (1991) Programmed
translational frameshifting and initiation at an AUU codon in gene expression
of bacterial insertion sequence IS911. J
Mol Biol 222: 465-477.
- Sekine Y & Ohtsubo E (1989)
Frameshifting is required for production of the transposase encoded by
insertion sequence 1. Proc Natl Acad Sci
U S A 86: 4609-4613.