MICROSPORIDIAN ENCEPHALITOZOON-CUNICULI, A UNICELLULAR EUKARYOTE WITHAN UNUSUAL CHROMOSOMAL DISPERSION OF RIBOSOMAL GENES AND A LSU RIBOSOMAL-RNA REDUCED TO THE UNIVERSAL CORE

Microsporidia are eukaryotic parasites lacking mitochondria, the ribos omes of which present prokaryote-like features. In order to better und erstand the structural evolution of rRNA molecules in microsporidia, t he 5S and rDNA genes were investigated in Encephalitozoon cuniculi. Th e genes are not in close proximity. Non-tandemly arranged rDNA units a re on every one of the fl chromosomes, Such a dispersion is also shown in two other Encephalitozoon species. Sequencing of the 5S rRNA codin g region reveals a 120 nt long RNA which folds according to the eukary otic consensus structural shape. In contrast, the LSU rRNA molecule is greatly reduced in length (2487 nt), This dramatic shortening is esse ntially due to truncation of divergent domains, most of them being rem oved, Most variable stems of the conserved core are also deleted, redu cing the LSU rRNA to only those structural features preserved in all l iving cells. This suggests that the E.cuniculi LSU rRNA performs only the basic mechanisms of translation. LSU rRNA phylogenetic analysis wi th the BASEML program favours a relatively recent origin of the fast e volving microsporidian lineage. Therefore, the prokaryote-like ribosom al features, such as the absence of ITS2, may be derived rather than p rimitive characters.