Hirochika H, Otsuki H
Department of Molecular Biology, National Institute of Agrobiological Resources, Ibaraki, Japan.
Gene. 1995 Nov 20;165(2):229-32. doi: 10.1016/0378-1119(95)00581-p.
Extrachromosomal DNA forms of Drosophila retrotransposons (RTn) and retroviruses have been extensively analyzed. However, no such analysis with plant RTn has been reported. Here, we report the analysis of extrachromosomal forms of the tobacco RTn Tto1. Tto1 is one of a few active RTn of plants and has been shown to be activated in tissue culture. Extrachromosomal circular DNA forms of Tto1, with one or two long terminal repeats (LTR), were found in cultured cells. Sequence analysis of the sites of circularization through joining two LTR showed that the junction between the LTR contains small deletions and/or insertions. The insertions are heterogeneous and do not show any homology to the Tto1 sequence. Similar insertions have been detected in the extrachromosomal circular forms of the copia element of Drosophila and suggested to be the result of excision of genomic copia. The structural features of the junctions found in Tto1 suggest that the insertions are produced by a mechanism other than excision. The potential mechanism of production of the extrachromosomal circular forms of Tto1 is discussed.
果蝇逆转录转座子（RTn）和逆转录病毒的染色体外DNA形式已得到广泛分析。然而，尚未有关于植物RTn的此类分析报道。在此，我们报告了对烟草RTn Tto1染色体外形式的分析。Tto1是植物中少数活跃的RTn之一，并且已证实在组织培养中会被激活。在培养细胞中发现了具有一个或两个长末端重复序列（LTR）的Tto1染色体外环状DNA形式。通过连接两个LTR对环化位点进行的序列分析表明，LTR之间的连接处包含小的缺失和/或插入。插入是异质的，并且与Tto1序列没有任何同源性。在果蝇的copia元件的染色体外环状形式中也检测到了类似的插入，并被认为是基因组copia切除的结果。在Tto1中发现的连接处的结构特征表明，这些插入是由切除以外的机制产生的。本文讨论了Tto1染色体外环状形式产生的潜在机制。
