Group I and Group II Introns and RNA Splicing

In the years 1981-1984, most of the research activity on introns was concentrated on RNA splicing. Two major discoveries were made: the distinction between group I and group II introns based on their proposed secondary structures, and the existence of RNA catalysis. My lab was highly involved in the first. Back in Gif-sur-Yvette in 1981,1 had started to examine the evolutionary conservation of the œ intron in other yeast species (see Sect. 5, below). Having joined my team, François Michel first noted that the sequences of some yeast mitochondrial introns (a handful) allowed the prediction of possible common RNA structures. The situation was complicated by the fact that the intronic reading frames are not always at the same place in the different introns. Among all possible hairpin structures, François Michel selected those that would be common to all introns. The logic was that the primary sequences need not be conserved between different introns so long as they allow the formation of equivalent double-stranded stems in a secondary structure model. The result was astonishing (Michel et al. 1982). Seven of the available yeast intron sequences shared a common secondary structure. The three others shared another structure. We decided to designate the first introns "group I" (the œ intron was among them) and the others "group II". This nomenclature is now universally used. Interestingly, the Tetrahymena intron, which had just been proven to be a ribozyme (Kruger et al. 1982), could also be folded in a secondary structure typical of group I introns (Michel and Dujon 1983). Using novel sequences from the fungus Emericella nidulans, a similar structural model was proposed for the group I introns by Davies et al. (1982), who proposed exon-intron pairings as a guide for RNA splicing (the so-called PI stem, critical for splice site recognition).

The a intron was a founding member of the group I introns, but was still the only one known to propagate in crosses. What was the origin of its "mobility"?

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