Alternative Splicing to Tissue Specific Splicing - An Evolutionary Pathway?
Tissue specific splicing of pre-messenger RNA is an important mechanism of regulating metazoan gene expression. There are scores of examples of single species of pre-messenger RNA which are synthesized in various tissues or in one tissue at various stages of development, but which are spliced to give slightly different collections of exons in each tissue or at each stage of development. We wish to contrast such tissue specific splicing to alternative splicing: in the latter, various collections of exons are spliced from the same pre-messenger RNA, but these different splicing events take place in the same cell at the same time. Alternative splicing, then, generates diversity, but not necessarily regulation. In this article, we shall argue that alternative splicing was a characteristic of all primitive splicing systems. We shall further argue that during evolution, the precision of splicing was increased and that this led to two different results. One was constitutive splicing, in which one and only one 5′ end of an intron is spliced to a unique 3′ end of this intron. The other was tissue specific splicing, in which the original ambiguity of primitive splicing was harnessed to the demands of differentiating tissues in metazoans.
KeywordsSplice Site Splice Reaction Splice System Spliceosome Assembly Branch Point Sequence
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