A novel ORF-containing group I intron with His-Cys box in the LSU rDNA of Naegleria

While group I introns are prevalent in the nuclear small subunit ribosomal DNA (SSU rDNA) in the amoeboflagellate genus Naegleria they have been detec ted in the nuclear large subunit ribosomal DNA (LSU rDNA) in only two linea ges of this genus, N. morganensis and an as yet unnamed Naegleria sp. In di fferent strains of the unnamed Naegleria sp. we have found different combin ations of the presence and absence of introns in the SSU rDNA and LSU rDNA. Furthermore, group I introns present in the LSU rDNA of these strains are of two different sizes. The shorter intron is 474 nt in length and has no o pen reading frame (ORF). The longer (868 nt) intron contains an ORF encodin g 143 amino acids with a His-Cys box. This ORF is different from the ORF (w hich encodes 175 amino acids) with His-Cys box in the second (919 nt) LSU r DNA group I intron of N. morganensis. The results of the phylogenetic analy ses of the His-Cys box in the ORFs support the hypothesis that the group I introns in the LSU rDNA of both N. morganensis and the unnamed Naegleria sp . were acquired by horizontal transfer. The different E26' locations of the short and the long group I intron in the unnamed Naegleria sp. supports th e hypothesis that they were also acquired separately by horizontal transfer .