The extrachromosomal elements of the Naegleria genus: How little we know

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. •Naegleria amoebae carry all ribosomal genes (rDNA) on closed circular extrachromosomal elements.•Each element encodes a single rDNA copy.•A region upstream of the 18S rRNA coding region is highly conserved between N. fowleri, N. lovanensis, and N. gruberi.•The region upstream of the 18S rRNA coding region is suggested to be involved in processing the rRNA transcript.•These elements contain a G-rich region which may contain the rRNA promoter.