There are currently 47 characterized species in the Naegleria genus of free-living amoebae. Each amoeba has thousands of extrachromosomal elements that are closed circular structures comprised of a single ribosomal DNA (rDNA) copy and a large non-rDNA sequence. Despite the presence of putative open reading frames and introns, ribosomal RNA is the only established transcript. A single origin of DNA replication (ori) has been mapped within the non-rDNA sequence for one species (N. gruberi), a finding that strongly indicates that these episomes replicate independently of the cell's chromosomal DNA component. This article reviews that which has been published about these interesting DNA elements and by analyzing available sequence data, discusses the possibility that different phylogenetically related clusters of Naegleria species individually conserve ori structures and suggests where the rRNA promoter and termination sites may be located.

目前有 47 个特征物种属于自由生活的变形虫Naegleria属。每个变形虫都有数以千计的染色体外元件，这些元件是封闭的环状结构，由单个核糖体 DNA (rDNA) 拷贝和一个大的非 rDNA 序列组成。尽管存在假定的开放阅读框和内含子，但核糖体 RNA 是唯一确定的转录本。一个物种（N. gruberi)，这一发现有力地表明这些附加体的复制独立于细胞的染色体 DNA 成分。本文回顾了已发表的关于这些有趣的 DNA 元件的文章，并通过分析可用的序列数据，讨论了Naegleria物种的不同系统发育相关簇单独保存 ori 结构的可能性，并建议 rRNA 启动子和终止位点可能位于何处。