Common features of hemiascomycetous introns
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- Hemiascomycetous yeast genomes are intron-poor genomes.
Only a very small proportion of nuclear genes (1 to 5%) are interrupted by an intronic structure
[Bon et al., 2003].
- Most hemiascomycetous yeast nuclear genes are mono-intronic genes.
As it occurs in S. cerevisiae nuclear genes [Spingola et al., 1999], nearly all hemiascomycetous counterparts (>96%) contain only one intron [Bon et al., 2003]. They can exceptionally contain up to two introns but never more.
- Introns are preferentially located at the 5'end of the ORFs.
About 92% of the introns reside inside an open reading frame. Most (more than 87%) are located near to the 5'end or within the first 100 nt following the start codon. External introns also exist but they remain rare (less than 8%). All reside in the 5' untranslated region, none in the 3'UTR untranslated region [Spingola et al., 1999; Bon et al., 2003].
- Ribosomal protein genes have always larger introns than non-ribosomal protein genes.
Intron size is not uniform within a given yeast species genome. It can vary from nearly 1000 nt down to 52 nt, generating very high standard deviations when mean intron size is calculated [Bon et al., 2003].
In fact, in S. cerevisiae [Rodriguez-Medina et al., 1994; Spingola et al., 1999] as in the 7 other hemiascomycetous species [Bon et al., 2003], intron size follows a bimodal distribution due to the existence of two populations of introns within a given yeast genome: short introns are preferentially located in non-ribosomal protein genes, and large introns are rather located in ribosomal protein genes.
Such bimodality is due to the variation in size of the distance between the 5' splice site and the branch site (S1 distance), and not to that of the distance between the branch site and the 3' splice site (S2 distance) which follows a unimodal distribution [Spingola et al., 1999; Bon et al., 2003].
The proportion of intron-containing ribosomal protein genes can vary from 28% up to 61%, depending on the yeast species [Bon et al., 2003].
- All hemiascomycetous yeast introns share a non-random vocabulary
Last modified: Tue Feb 10 11:08:09 CET 2004