HAU16008-03-3 - NATURE|Vol 446|5 April 2007 NEWS VIEWS The...

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Aux/IAA repressors Aux/IAA destruction SCF TIR1 complex TIR1 Low auxin High auxin a b Auxin-response- gene repression ARF activators Auxin-response- gene activation The authors argue that further measurements with improved precision will allow tunnelling to be characterized more fully. Such measure- ments will require X-ray pulses significantly shorter than those of a few hundred attosec- onds that are presently available. Once this hurdle has been surmounted, the tunnelling method could be used to probe how electrons behave in a wide range of atomic and molecular systems with attosecond precision. In the meantime, Uiberacker et al. turn their attention 1 to the significantly slower processes associated with the decay of excited states in the xenon ions Xe 2+ and Xe 3+ . Using their laser-field tunnelling method to produce Xe 4+ ions, they have been able to measure cascaded population transfer processes among the elec- tron states of ions that have lifetimes of as little as 6 femtoseconds (6 × 10 –15 s). They thus already show the potential of their method to extend and increase the precision of findings estab- lished in earlier spectroscopic studies. Jonathan P. Marangos is in the Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2BW, UK. e-mail: [email protected] 1. Uiberacker, M. et al. Nature 446, 627–632 (2007). 2. Baltuska, A. et al. Nature 421, 611–615 (2003). 3. Sansone, G. et al. Science 314, 443–446 (2006). 4. Baker, S. et al. Science 312, 424–427 (2006). 5. Drescher, M. et al. Nature 419, 803–807 (2002). 6. Oppenheimer, J. R. Phys. Rev. 31, 66–81 (1928). repressors, are recruited to the receptor in an auxin-dependent manner and, after binding to TIR1, are degraded. Identification of the TIR1 receptor suggested that auxin perception and the signalling pathway to auxin-regulated gene expression was direct and simple, but it left various questions. Tan et al. 2 now describe crystal structures of a TIR1 complex that reveal how auxins fit into a surface pocket of TIR1 and enhance the binding of Aux/IAA repressors to TIR1. Auxin-regulated gene expression triggers most of the processes controlled by this plant hormone. Many auxin-induced genes are regu- lated by the interplay of two classes of gene- transcription factors, auxin-response factors (ARFs) and the Aux/IAA repressors 5 (Fig. 1). ARFs bind to auxin-response promoter ele- ments in auxin-response genes. When auxin concentrations are below a threshold level, Aux/IAA repressors associate with ARF acti- vators and repress the expression of these genes. Conversely, higher levels of auxin lead to destruction of the Aux/IAA repressors, and to activation of the genes 6,7 . Aux/IAA repressors contain four conserved domains, and one of these, domain II, is respon- sible for the instability of these proteins (Fig. 2a, overleaf). Domain II has a hallmark GWPPV amino-acid motif that is recognized by TIR1 in the SCF TIR1 complex 8,9 . IAA binds to TIR1 to enhance the recruitment of the motif to TIR1, as do two synthetic auxins — 1-naphthalene acetic acid (1-NAA) and 2,4-dichlorophenoxy-
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