Kelly R J, Siegel A
Department of Biological Sciences, Wayne State University, Detroit, MI 48202.
Gene. 1989 Aug 15;80(2):239-48. doi: 10.1016/0378-1119(89)90288-6.
The nucleotide (nt) sequence of the 5508-nt intergenic spacer (IGS), between the 25S- and the 18S-coding regions of Cucurbita maxima rDNA, was determined. The fragment sequenced is 6142 nt long and includes 472 nt of 25S- and 162 nt of 18S-coding regions. The IGS has a complex primary structure, composed of five repetitive families (A-E) and three unique domains. It is dominated by the presence of nine, tandemly-repeating units of approximately 250 nt (repeat D), each unit containing four copies of an internal subrepeat (repeat E). The repetitive units show sequence variability consisting of nt changes, insertions and deletions. Upstream of the nine D repeats and between two copies of the B repeat is a 575-nt region, highly G + C rich (83%) and heavily biased toward C (58%) in the sense strand. Within this region are six repetitive units, averaging 42 nt (repeat C) each, containing but a single A nt. Downstream from the terminus of the 25S-coding sequence, are two tandem copies of the 103-nt A repeat. The IGS of C. maxima is longer and more complex than that of other plant IGSs described to date. The 600 nt at the 5' portion of cucurbit IGS is more conserved in evolution than the remainder, as revealed by comparison of C. maxima and C. pepo IGS restriction maps and by nucleotide sequence comparison of C. maxima and Cucumis sativa IGSs.
测定了南瓜属大果南瓜核糖体DNA（rDNA）25S编码区和18S编码区之间5508个核苷酸（nt）的基因间隔区（IGS）的核苷酸序列。测序的片段长6142 nt，包括25S编码区的472 nt和18S编码区的162 nt。IGS具有复杂的一级结构，由五个重复家族（A - E）和三个独特结构域组成。其主要特征是存在九个约250 nt的串联重复单元（重复序列D），每个单元包含四个内部亚重复序列（重复序列E）的拷贝。这些重复单元表现出由核苷酸变化、插入和缺失组成的序列变异性。在九个D重复序列的上游以及B重复序列的两个拷贝之间，有一个575 nt的区域，富含G + C（83%），正义链中C的含量极高（58%）。在该区域内有六个重复单元，平均每个42 nt（重复序列C），每个单元仅含一个A核苷酸。在25S编码序列末端的下游，是103 nt的A重复序列的两个串联拷贝。大果南瓜的IGS比迄今为止描述的其他植物IGS更长且更复杂。通过比较大果南瓜和西葫芦的IGS限制酶切图谱以及大果南瓜和黄瓜的IGS核苷酸序列，发现葫芦科IGS 5'端的600 nt在进化上比其余部分更保守。
