Clark D J, Bilanchone V W, Haywood L J, Dildine S L, Sandmeyer S B
Department of Microbiology and Molecular Genetics, College of Medicine, University of California, Irvine 92717.
J Biol Chem. 1988 Jan 25;263(3):1413-23.
Sigma is a 340- or 341-base pair repetitive element which is located almost exclusively within 19 base pairs of the 5' ends of various tRNA genes in the Saccharomyces cerevisiae genome. Although most sigma elements characterized to date are isolated insertions, a few of the elements occur relatively closely spaced. One of these pairs is a direct repeat of the sigma element separated by an internal domain 4.7 kilobase pairs in length. Not only does this structure resemble a composite transposable element, but regions within the sigma elements and intervening domain are homologous to conserved regions in retroviruses and retrotransposons of yeast and other organisms. Two features suggest that the sigma elements and intervening DNA transposed in a concerted event: only one of the two sigma elements is associated with a tRNA gene, and only the outside ends of the two elements are flanked by the 5-base pair direct repeats that usually flank individual sigma insertions. Examination of genomic DNA from five laboratory strains indicates that the 4.7 kilobase pair internal domain is present in one to four copies per haploid genome and that the genomic location of this domain differs from strain to strain. In addition, Northern blot analysis showed the presence of a 5.2 kilobase poly(A) transcript which hybridizes to both sigma and internal domain-specific probes. The existence of this composite element may suggest new ways to consider the mechanisms by which retrotransposons select their targets.
西格玛（Sigma）是一种由340或341个碱基对组成的重复元件，几乎只存在于酿酒酵母基因组中各种tRNA基因5'端的19个碱基对范围内。尽管迄今为止所鉴定的大多数西格玛元件都是孤立的插入片段，但其中一些元件的出现位置相对较近。其中一对是西格玛元件的直接重复序列，中间由一个长度为4.7千碱基对的内部结构域隔开。这种结构不仅类似于复合转座元件，而且西格玛元件内部区域和中间结构域与酵母及其他生物体逆转录病毒和反转录转座子中的保守区域同源。有两个特征表明西格玛元件和中间DNA是协同转座的：两个西格玛元件中只有一个与tRNA基因相关，并且两个元件的外侧末端仅由通常位于单个西格玛插入片段两侧的5个碱基对直接重复序列侧翼。对五个实验室菌株的基因组DNA进行检测表明，每个单倍体基因组中4.7千碱基对的内部结构域存在1至4个拷贝，并且该结构域的基因组位置因菌株而异。此外，Northern印迹分析显示存在一个5.2千碱基的聚腺苷酸化转录本，它与西格玛元件和内部结构域特异性探针均杂交。这种复合元件的存在可能为思考反转录转座子选择其靶标的机制提供新的途径。
