Grandbastien M-A, Audeon C, Bonnivard E, Casacuberta J M, Chalhoub B, Costa A-P P, Le Q H, Melayah D, Petit M, Poncet C, Tam S M, Van Sluys M-A, Mhiri C
Laboratoire de Biologie Cellulaire, INRA, Centre de Versailles, Versailles, France.
Cytogenet Genome Res. 2005;110(1-4):229-41. doi: 10.1159/000084957.
Tnt1 elements are a superfamily of LTR-retrotransposons distributed in the Solanaceae plant family and represent good model systems for studying regulatory and evolutionary controls established between hosts and transposable elements. Tnt1 retrotransposons tightly control their activation, by restricting expression to specific conditions. The Tnt1A element, originally discovered in tobacco, is expressed in response to stress, and its activation by microbial factors is followed by amplification, demonstrating that factors of pathogen origin can generate genetic diversity in plants. The Tnt1A promoter has the potential to be activated by various biotic and abiotic stimuli but a number of these are specifically repressed in tobacco and are revealed only when the LTR promoter is placed in a heterologous context. We propose that a tobacco- and stimulus-specific repression has been established in order to minimize activation in conditions that might generate germinal transposition. In addition to tight transcriptional controls, Tnt1A retrotransposons self-regulate their activity through gradual generation of defective copies that have reduced transcriptional activity. Tnt1 retrotransposons found in various Solanaceae species are characterized by a high level of variability in the LTR sequences involved in transcription, and have evolved by gaining new expression patterns, mostly associated with responses to diverse stress conditions. Tnt1A insertions associated with genic regions are initially favored but seem subsequently counter-selected, while insertions in repetitive DNA are maintained. On the other hand, amplification and loss of insertions may result from more brutal occurrences, as suggested by the large restructuring of Tnt1 populations observed in tobacco compared to each of its parental species. The distribution of Tnt1 elements thus appears as a dynamic flux, with amplification counterbalanced by loss of insertions. Tnt1 insertion polymorphisms are too high to reveal species relationships in the Nicotiana genus, but can be used to evaluate species relationships in the Lycopersicon and Capsicum genera. This also demonstrates that the behavior of Tnt1 retrotransposons differs between host species, most probably in correlation to differences in expression conditions and in the evolutionary and environmental history of each host.
Tnt1元件是长末端重复序列反转录转座子超家族，分布于茄科植物家族，是研究宿主与转座元件之间建立的调控和进化控制的良好模型系统。Tnt1反转录转座子通过将表达限制在特定条件下，严格控制其激活。最初在烟草中发现的Tnt1A元件在受到胁迫时表达，其被微生物因子激活后会发生扩增，这表明病原体来源的因子可以在植物中产生遗传多样性。Tnt1A启动子有可能被各种生物和非生物刺激激活，但其中一些在烟草中被特异性抑制，只有当长末端重复序列启动子置于异源背景下时才会显现出来。我们认为，已经建立了烟草和刺激特异性抑制机制，以尽量减少在可能产生生殖系转座的条件下的激活。除了严格的转录控制外，Tnt1A反转录转座子还通过逐渐产生转录活性降低的缺陷拷贝来自我调节其活性。在各种茄科物种中发现的Tnt1反转录转座子的特征是，参与转录的长末端重复序列具有高度变异性，并通过获得新的表达模式而进化，这些新表达模式大多与对各种胁迫条件的反应相关。与基因区域相关的Tnt1A插入最初受到青睐，但随后似乎被反向选择，而在重复DNA中的插入则得以保留。另一方面，插入的扩增和丢失可能是由更剧烈的事件导致的，正如与烟草的每个亲本物种相比，在烟草中观察到的Tnt1群体的大规模重组所表明的那样。因此，Tnt1元件的分布似乎是一种动态通量，扩增与插入的丢失相互平衡。Tnt1插入多态性太高，无法揭示烟草属中的物种关系，但可用于评估番茄属和辣椒属中的物种关系。这也表明，Tnt1反转录转座子在宿主物种之间的行为不同，很可能与每个宿主的表达条件以及进化和环境历史的差异有关。
