NUCLEAR EXPRESSION OF A GROUP II INTRON

Nuclear expression of group II introns is consistent with spliceosomal introns ancestry

Nuclear expression of group II introns is consistent with spliceosomal introns ancestry

The leading hypothesis for the origin of spliceosomedependent introns is that they and the splice some it were derived from one or more self-splicing group II introns. Such introns have been found in the chromosomes and plasmids of numerous eubacteria as well as in one member of the archaea (S Zimmerly, personal communication). (Mechanically talking, although, some of the eubacterial assembly II elements are not introns at all, in that they lie between rather than inside genes.) As group II introns are also present in the organelles of plants, fungi, and protists (but not animals), they are ordered candidates for the primordial spliceosomal introns. The group II-origin hypothesis derives support from a hitting list of purposeful and purposeful likenesses between assembly II introns and the spliceosomal processing of atomic introns.

If this hypothesis is correct, the transition from self-splicing group II introns of eubacterial origin to a large population of eukaryotic snRNA-dependent introns would have involved several evolutionary challenges. First, a cohesive self-splicing scheme would have to give rise to a complicated fragmented spliceosomal system. Second, the fragmented constituents would have to keep their functionality while cooperatively functioning in trans. Third; a new set of splicing proteins would have to be employed to pattern the full spliceosomal complex. Fourth and finally, the derived population of splice some-dependent introns would have to expand and displace the ancestral population of self-splicing introns. Because there may be chemical constraints on how splicing can be carried out, structural and mechanistic similarities do not prove that spliceosomal introns arose from ancestral group II introns, but a variety of observations demonstrate the feasibility of the necessary steps. First, fragmented group II introns are common in eubacteria, and although none of these have been demonstrated to be functional, plant organelles do harbor group II fragments that successfully cooperate in splicing. Second, group II introns often encode their own reverse transcriptase, which assists in splicing and facilitates the explicative movement to new locations within the host genome by reverse splicing. Third, phylo-genetic analysis of assembly II introns suggests that they have transferred level amidst eubacterial species and possibly between microbes and eukaryotic organelles. Given the numerous renowned cases of organelle to atomic transfer of DNA, some movement to the atomic genome by this path is virtually guaranteed. Finally, because all group II introns utilize protein factors to facilitate splicing, some encoded in the nuclear genome the recruitment of such refinements to spliceosomal introns is expected. Given the tendency for group II introns to both fragment and mobilize, Stoltzfus suggested a mechanism by which the process of spliceosomal introns evolution might come about. He argues that once a set of cooperative trans-acting fragments became completely established, any assembly II introns residual in the atomic genome would have become subject to eventual decrease of their self-splicing adeptness by mutation so long as ...