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RNA processing

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RNA species in cells

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RNA processing

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Types of introns and their removal

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Types of introns

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Complex organisms have intron-rich genes

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Examples of introns in human protein-coding genes

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Removal of introns from mRNA in the nucleus of eukaryotes

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Consensus elements of human GU-AG introns

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Chemistry of mRNA splicing for GU-AG introns

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Three way junction at the internal (A) branch site

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U1 snRNP

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Spliceosome-catalyzed splicing of GU-AG introns

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Different snRNPs and proteins bind and dissociate during splicing

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Similarities in spliceosome-catalyzed removal of introns and self-splicing group I and group II introns

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Possible errors in splice-site selection

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SR proteins are essential for correct splice-site selection ESE = exonic splicing enhancers SR proteins = serine, arginine-rich proteins

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Trans-splicing joins exons from different RNAs

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Splicing of AU-AC introns by minor spliceosomes proceeds as for GU-AG introns

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Alternative splicing results in alternative mRNAs that are translated into different proteins

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Figure 12.34 Genomes 3 ( Garland Science 2007) Alternative splicing of the human slo gene (codes for a membrane protein that regulates the flow of potassium ions across the membrane) Optional exons participating in alternative splicing events. About 500 mRNA variants are produced in cells of the inner ear.

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Alternative splicing is regulated

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Assembly of major and minor spliceosomes Nonsense-mediated transcript decay

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Example: alternative splicing in Drosophila Dscam transcripts

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Example: mutual exclusive alternative splicing of exon 6 in Drosophila Dscam transcripts

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Splicing activators and repressors

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Exons may code for protein domains

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Exon shuffling may lead to protein diversity

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Exon shuffling may be responsible for protein diversity

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Figure 12.40 Genomes 3 ( Garland Science 2007) Removal of introns from tRNAs

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RNA processing

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Processing of ribosomal RNA by cutting and trimming

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Figure 12.17 Genomes 3 ( Garland Science 2007) Processing of a tRNA by cutting and trimming

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RNA processing

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Figure 12.18 Genomes 3 ( Garland Science 2007) Chemical modifications in tRNAs

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Figure 12.41a Genomes 3 ( Garland Science 2007) Methylation of ribosomal RNAs

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RNA editing in apolipoprotein B mRNA

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Deamination of bases leads to codon changes

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RNA-guided insertion of Us into mRNAs

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Processing of RNAs is required for transport into the cytoplasm