Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, Nebraska 68198, USA.
Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, Nebraska 68198, USA.
Protist. 2019 Apr;170(2):141-152. doi: 10.1016/j.protis.2019.02.001. Epub 2019 Feb 13.
The genes encoding the ribosomal RNA (rRNA) subunits of the amoeba Naegleria gruberi are encoded in a relatively uncommon arrangement: on a circular extrachromosomal DNA element with each organism carrying about 4,000 copies of the element. As complete sequence analysis of the N. gruberi chromosomal DNA revealed no copy of the rRNA genes, these extrachromosomal elements must therefore replicate autonomously. We reported elsewhere the molecular cloning and the complete sequence analysis of the entire rRNA gene-containing element of N. gruberi (strain EG). Using neutral/neutral two-dimensional agarose electrophoresis, the region in the element enclosing the single replication origin using DNA from asynchronous and axenically propagated N. gruberi populations was localized within a 2.1 kbp fragment located approximately 2,300bp from the 18S rRNA gene and 3,700bp from the 28S rRNA gene. The results indicate that replication occurs from a single origin via a theta-type mode of replication rather than by a rolling circle mode. Further, G-quadruplex elements, often located near DNA replication origins, occur in and near this fragment in a repeated sequence.
嗜热四膜虫核糖体 RNA（rRNA）亚基的基因编码以一种相对不常见的方式排列：在一个圆形的染色体外 DNA 元件上，每个生物体携带大约 4000 个该元件的拷贝。由于对嗜热四膜虫染色体 DNA 的完整序列分析没有发现 rRNA 基因的任何拷贝，因此这些染色体外元件必须自主复制。我们在其他地方报道了嗜热四膜虫（EG 株）完整的 rRNA 基因包含元件的分子克隆和完整序列分析。使用中性/中性二维琼脂糖电泳，从非同步和无共生繁殖的嗜热四膜虫群体的 DNA 中，对包含单个复制起点的元件区域进行了定位，该区域位于大约 18S rRNA 基因 2300bp 和 28S rRNA 基因 3700bp 处。结果表明，复制是从单个原点通过θ型复制模式而不是滚环模式发生的。此外，经常位于 DNA 复制起点附近的 G-四链体元件出现在该片段内和附近的重复序列中。
