Introns class

Although spliceosomal introns appear to be limited to eukaryotes, the other intron classes are not. Genes with group I and II introns have now been found in both bacteria and bacterial viruses. Bacteriophage T4, for example, has several protein-encoding genes with group I introns. Introns appear to be more common in archae-bacteria than in eubacteria. 

Recently, a novel class of type 1-like human IFNs, named 1FN-A,1 or lL-29,1FN-A.2 (1F-28A) and 1FN-X3 (1F-28B), was identified (Dumoutier et al. 2003 Sheppard et al. 2003). The three IFN-A, genes cluster on human chromosome 19 and comprise 5 exons for 1FN-A,1 and 6 for 1FN-A.2 and 1FN-A.3, and several introns (Table 1). They encode 20- to 22-kDa secreted monomeric proteins of 196 to 200 amino acids. Type 111 IFNs have also been identified in other species such as mice, birds, and fish. 

The fourth class of introns, found in certain tRNAs, is distinguished from the group I and II introns in that the splicing reaction requires ATP and an endonuclease. The splicing endonuclease cleaves the phosphodiester bonds at both ends of the intron, and the two exons are joined by a mechanism similar to the DNA ligase reaction (see Fig. 25-16). 

Fig. 6. Lengths of introns and exons within the coding region of human 5-HT receptors. Exons are shown in white background introns are shown in black background. The 5-HTj class of receptors has a single exon, and is not shown.

Finally, paternally inherited sequences, whether introns or exons of a gene, provide a distinct new class of characters for use in phytogeny reconstruction, as they represent one of the three modes of inheritance paternal, maternal, and Mendelian. For this reason, the use of paternally inherited Y chromosome sequences as character data in phylogenetic analyses will add a new dimension to our understanding of the evolutionary history of mammalian taxa. 

Design of RNA molecules with novel catalytic functions called ribozymes (ribonucleotide enzymes) started out from the reprogramming of naturally occurring molecules to accept unnatural substrates [32, 33] A specific RNA cleaving ribozyme, a class I (self-splicing) intron, was modified through variation and selection until it operated efficiently on DNA. The evolutionary path of such a transformation of catalytic activity has been recorded in molecular detail [34]. The basic problem in the evolutionary design of new catalysts is the availability of appropriate analytical tools for the detec-... 

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