Nucleic Acids Research, Vol 27, Issue 23 4533-4540, Copyright © 1999 by Oxford University Press
N Joseph, E Krauskopf, MI Vera and B Michot
Molecular mechanisms of ITS2 processing, a eukaryotic insertion between the
5.8S and LSU rRNA, remain largely elusive even in yeast. To delineate ITS2
structural and functional features which could be common to eukaryotes, we
first produced phylo-genetically supported folding models in the vertebrate
lineage, then tested them in deeper branchings and, more particularly,
among yeasts. ITS2 comparisons between four Teleostei, a Chondrichthyes
specimen and two jawless organisms have revealed a common folding
architecture in four to five domains of secondary structure emerging from a
preserved structural core. This folding, largely reminiscent of ITS2
architecture in mammals, is also preserved in amphibia and in chicken,
despite dramatic sequence variations. Preferential conservation is located
around a central loop and at the apex of a long stem in the ITS2 3'-half.
Interestingly, these two independent structural features contain,
respectively, the 3'- ends of the two transient rRNA precursors 8S and 12S
RNA identified in mammals, suggesting a preservation of these intermediates
of processing over the entire vertebrate group. Surprising similarities
between the vertebrate ITS2 folding shape and that of invertebrates as well
as protista have made intriguing the significant differences from the yeast
model. A detailed comparative analysis including four relatively close
species and Schizosaccharomyces pombe, a deep yeast branching, has revealed
an alternative phylogenetically supported four-domain folding presenting
strong similarities to the vertebrate model. Remarkably, the two best
conserved regions of vertebrates have unambiguously preserved counterparts
which are also sites for internal processing in yeast. Therefore, molecular
mechanisms involved in ITS2 excision in vertebrates and yeast might be more
closely related than currently believed and might require a very similar
trans -acting machinery.
ARTICLES
Ribosomal internal transcribed spacer 2 (ITS2) exhibits a common core of secondary structure in vertebrates and yeast
Laboratoire de Biologie Moleculaire Eucaryote du CNRS, Universite Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France.
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