Genes Within Introns

In retrospect, since many group I introns are self-splicing, one might expect to find that the mutations that lead to processing defects would disrupt essential RNA structural elements required for folding and/or catalysis. However, genetic and sequencing analysis of three group I introns in the S. cerevisiae cytochrome b (cob) gene revealed a number of surprising features. First, the introns contain long coding regions in frame with the upstream 5' exon ORFs. Second, many of the splicing impaired mutants carry non-sense codons in the intron ORF, suggesting that the full-length fusion protein or ribosome read through is required for splicing. Thirdly, disruption of upstream splicing events prevents expression of downstream intron ORFs, and thus, splicing of those introns. Finally, it was shown that mutants are rescued by providing the wild-type intron-encoding protein in trans (Lazowska et al. 1980,1989; Grou-dinsky et al. 1981; Anziano et al. 1982; Weiss-Brummer et al. 1982; Delahodde et al. 1989). These observations provided a convincing case that intron-encod-ed proteins are required for group I intron splicing in vivo and were appropriately named "maturases".

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