The development of designed site-specific endonucleases boosted the establishment of gene targeting (GT) techniques in a row of different species. However, the methods described in plants require a highly efficient transformation and regeneration procedure and, therefore, can be applied to very few species. Here, we describe a highly efficient GT system that is suitable for all transformable plants regardless of transformation efficiency. Efficient in planta GT was achieved in Arabidopsis thaliana by expression of a sitespecific endonuclease that not only cuts within the target but also the chromosomal transgenic donor, leading to an excised targeting vector. Progeny clonal for the targeted allele could be obtained directly by harvesting seeds. Targeted events could be identified up to approximately once per 100 seeds depending on the target donor combination. Molecular analysis demonstrated that, in almost all events, homologous recombination occurred at both ends of the break. No ectopic integration of the GT vector was found.plant biotechnology | plant breeding | gene technology | double-strand-break repair S ince the first report on gene targeting (GT) in plants was published (1), various approaches were tested to improve the efficiency of the method (2-5), which has been summarized in recent reviews (6, 7). We were able to demonstrate that the integration of a transfer DNA (T-DNA) by homologous recombination (HR) into a specific locus could be enhanced by two orders of magnitude via double-strand-break (DSB) induction using a site-specific endonuclease (8). More recently, by the use of zinc finger nucleases (9), which, in principle, can be used to induce a DSB at any genomic site, endogenous loci have been targeted in Arabidopsis (10), tobacco (11), and maize (12) at high frequencies (13). Some time ago, a method for in vivo targeting was developed in Drosophila. A stably integrated donor precursor molecule is first circularized by the FLP recombinase and subsequently linearized via cutting a single I-SceI recognition site, generating the actual GT vector (14). However, such a technique has not been successfully transferred to plants. We have previously shown that DNA can be efficiently excised from the genome in planta by the use of a site-specific endonuclease (15). To test whether the combination of this approach with DSB-induced recombination might lead to an efficient GT system, that is independent of transformation, we performed a proof-of-concept (POC) experiment in Arabidopsis using I-SceI (16) as a sitespecific nuclease. ResultsGenerating Homozygous Single-Copy GT Lines. Our in planta GT system is based on three different constructs (two shown in Fig. 1A) that were transformed independently by floral dipping. The target locus contains a truncated β-glucuronidase (GUS) gene (uidA) that can be restored via GT. DSB induction at the two ISceI recognition sites flanking a kanamycin-resistance gene would result in excision of the kanamycin-resistance gene and in activation of the target locus for HR. Th...

SUMMARYIn recent years, multiple factors involved in DNA double-strand break (DSB) repair have been characterised in Arabidopsis thaliana. Using homologous sequences in somatic cells, DSBs are mainly repaired by two different pathways: synthesis-dependent strand annealing (SDSA) and single-strand annealing (SSA). By applying recombination substrates in which recombination is initiated by the induction of a site-specific DSB by the homing endonuclease I-SceI, we were able to characterise the involvement of different factors in both pathways. The nucleases MRE11 and COM1, both involved in DSB end processing, were not required for either SDSA or SSA in our assay system. Both SDSA and SSA were even more efficient without MRE11, in accordance with the fact that a loss of MRE11 might negatively affect the efficiency of non-homologous end joining. Loss of the classical recombinase RAD51 or its two paralogues RAD51C and XRCC3, as well as the SWI2/SNF2 remodelling factor RAD54, resulted in a drastic deficiency in SDSA but had hardly any influence on SSA, confirming that a strand exchange reaction is only required for SDSA. The helicase FANCM, which is postulated to be involved in the stabilisation of recombination intermediates, is surprisingly not only needed for SDSA but to a lesser extent also for SSA. Both SSA and SDSA were affected only weakly when the SMC6B protein, implicated in sister chromatid recombination, was absent, indicating that SSA and SDSA are in most cases intrachromatid recombination reactions.

The twin-arginine translocase (Tat) provides protein export in bacteria and plant chloroplasts and is capable of transporting fully folded proteins across the membrane. We resolved the conformation and membrane alignment of the pore-forming subunit TatA(d) from Bacillus subtilis using solid-state NMR spectroscopy. The relevant structured part of the protein, TatA(2-45), contains a transmembrane segment (TMS) and an amphiphilic helix (APH). It was reconstituted in planar bicelles, which represent the lipid environment of a bacterial membrane. The SAMMY solid-state NMR experiment was used to correlate (15)N chemical shifts and (1)H-(15)N dipolar couplings in the backbone and side chains of the (15)N-labeled protein. The observed wheel-like patterns ("PISA wheels") in the resulting 2-dimensional spectra confirm the α-helical character of the two segments and reveal their alignment in the lipid bilayer. Helix tilt angles (τ(TMS) = 13°, τ(APH) = 64°) were obtained from uniformly labeled protein, and azimuthal rotations (ρ(Val15) = 235°, ρ(Ile29) = 25°) were obtained from selective labels. These constraints define two distinct families of allowed structures for TatA in the membrane-bound state. The manifold of solutions could be narrowed down to a unique structure by using input from a liquid-state NMR study of TatA in detergent micelles, as recently described [Hu, Y.; Zhao, E.; Li, H.; Xia, B.; Jin, C. J. Am. Chem. Soc. 2010, DOI: 10.1021/ja1053785]. Interestingly, the APH showed an unexpectedly slanted alignment in the protein, different from that of the isolated APH peptide. This finding implies that the amphiphilic region of TatA is not just a flexible attachment to the transmembrane anchor but might be able to form intra- or even intermolecular salt-bridges, which could play a key role in pore assembly.

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