Hemiascomycetous yeast introns



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Introns are sequences present in various types of genes that need to be removed from primary transcripts to allow the formation of functional RNAs. Introns are present in all classes of RNA (rRNA, tRNA, mRNA, etc?) and have been found in various genomes (eukaryotic, prokaryotic, organelles, viruses, etc?). Intron sequences have to be precisely recognized and eliminated from pre-RNA to allow for functional protein or RNA synthesis. In a few cases, introns are involved in the regulation of the expression of their host genes, are alternatively spliced, correspond to mobile genetic elements or code themselves for protein or functional RNA [1].

Major classes of introns

At least five distinct classes of introns can be distinguished in hemiascomycetous yeasts on the basis of their mechanism of splicing and/or characteristic sequences:

Host genomeIntron classIntronsGenesIntron/GeneS. cerevisiae genes
NucleusPre-mRNA2702631 or 2~ 5978(a)
tRNA61611274
HAC1111~ 5978(a)
MitochondriaGroup I931 - 42
Group II421 - 38
(a) - ORFs except questionable ORFs.
  1. Nuclear pre-mRNA introns or spliceosomal introns: Are found within the nucleus of yeasts, interrupting the pre-mRNAs of protein-coding genes. Nuclear pre-mRNA introns are known as spliceosomal introns because of the machinery used to excise them from pre-mRNAs and which is called the spliceosome. Splicing of spliceosomal introns, or conventional pre-mRNA introns, occurs by means of a lariat intermediate via two trans-esterification reactions. [1, 2].


  2. HAC1 intron: Is a non-conventional pre-mRNA intron found in the S. cerevisiae HAC1/ERN4 nuclear gene, which encodes a transcription factor responsible for the unfolded protein response. Splicing of the HAC1 intron occurs by a novel pathway, bypassing the spliceosome, and is highly regulated. Splicing is catalysed by protein factors, some of which being shared with tRNA introns (i.e. the tRNA ligase). The cleavage of the splice sites is sequence-specific and occurs in a random order, and the branchpoint-like sequence is not required for splicing [1, 3-4].


  3. tRNAs introns: Are found in nuclear tRNA genes. About one-fifth of the yeast tRNAs contain introns. Splicing of tRNA introns is catalysed by the sequential action of three protein enzymes: a site-specific endonuclease, a tRNA ligase, and a phosphotransferase [1, 5-6].


  4. Group I introns: Are found in certain mitochondrial mRNAs and rRNAs of yeasts. They are mobile and most have the ability to splice themselves (self-splicing) [1, 7-10].


  5. Group II introns: Are found within certain mitochondrial mRNAs of yeasts. Some are mobile and have the ability to splice themselves (self-splicing). but protein machinery is required in vivo. Splicing of group II introns occurs via a lariat intermediate and two trans-esterification reactions similar to the ones occurring for nuclear pre-mRNA [1, 10-13].

Selected references:

More details can be found in the references cited below:
Last modified: Tue Feb 10 11:14:18 CET 2004