Abstract
We have amplified the large subunit ribosomal DNA (LSUrDNA) of the 12 described Naegleria spp. and of 34 other Naegleria lineages that might be distinct species. Two strains yielded a product that is longer than 3 kb, which is the length of the LSUrDNA of all described Naegleria spp. Sequencing data revealed that the insert in one of these strains is a group I intron without an open reading frame (ORF), while the other strain contains two different group I introns, of which the second intron has an ORF of 175 amino acids. In the latter ORF there is a conserved His-Cys box, as in the homing endonucleases present in group I introns in the small subunit ribosomal DNA (SSUrDNA) of Naegleria spp. Although the group I introns in the LSUrDNA differ in sequence, they are more related to each other than they are to the group I introns in the SSUrDNA of Naegleria spp. The three group I introns in the LSUrDNA in Naegleria are at different locations and are probably acquired by horizontal transfer, contrary to the SSUrDNA group I introns in this genus which are of ancestral origin and are transmitted vertically.