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• W1490999666 abstract "The removal of non-coding sequences, introns, is an essential part of messenger RNA processing. In most metazoan organisms, the U12-type spliceosome processes a subset of introns containing highly conserved recognition sequences. U12-type introns constitute less than 0,5% of all introns and reside preferentially in genes related to information processing functions, as opposed to genes encoding for metabolic enzymes. It has previously been shown that the excision of U12-type introns is inefficient compared to that of U2-type introns, supporting the model that these introns could provide a rate-limiting control for gene expression. In this work, cells with low abundance of the U12-type spliceosome were found to inefficiently process U12-type introns encoded by a transfected construct, but with endogenous genes, the abundance of the U12-type spliceosome was not found to affect expression levels. However, significant levels of endogenous unspliced U12-type introncontaining pre-mRNAs were detected in cells. Together these results support the idea that U12-type splicing may limit gene expression in some situations. The effect of U12-type splicing efficiency on a whole organism was studied in a Drosophila mutant deficient in U12-type splicing. Genes containing U12-type introns showed variable gene-specific responses to the splicing defect. Surprisingly, microarray screening revealed that metabolic genes were enriched among downstream effects, and that the phenotype could largely be attributed to one U12-type intron-containing mitochondrial gene. Gene expression control by the U12-type spliceosome could thus have widespread effects on metabolic functions in the organism. The subcellular localization of the U12-type spliceosome components was studied as a response to a recent dispute on the localization of the U12-type spliceosome. All components studied were found to be nuclear indicating that the processing of U12-type introns occurs within the nucleus, thus clarifying a question central to the field. Review of the literature 1 Review of the literature 1.1 Overview of eukaryotic pre-mRNA processing A eukaryotic gene typically consists of several stretches of coding sequence, exons, separated by non-coding introns (Figure 1). The discontinuous structure of eukaryotic genes presents a problem for the gene expression machinery: open reading frames separated by intronic areas must be joined together to produce a functional messenger RNA (mRNA). This process, mRNA splicing, is achieved by the spliceosome, a large complex of small nuclear RNA (snRNA) molecules assembled into small nuclear ribonucleoprotein (snRNP) particles and numerous auxiliary proteins. Intron recognition and spliceosome assembly are followed by catalytic reactions that result in the joining of exons and release of the intron. Splicing occurs primarily cotranscriptionally, and is integrated with other pre-mRNA processing events in the nucleus, such as capping and polyadenylation. Transcripts become mature mRNAs through multiple processing steps prior to transport out of the nucleus into the cytoplasm, where they can be translated, stored or degraded. 1.2 Introns in eukaryotic genes 1.2.1 Complex organisms have large variation of intron sizes In higher eukaryotes, multiple introns are present in most genes. Mammals in particular have a surprising variety of intron sizes and numbers per gene. In humans, mean intron length is 3300 bp, whereas in the fruit fly Drosophila melanogaster, the average length is less than 500 bp (Lander et al. 2001). An extreme example is provided by the longest human gene, coding for the muscle-specific protein dystrophin. It spans 2,5 million bases, but its 79 exons account less than 1% of total length, with the average intron length of 26 000 bases (Pozzoli et al. 2002). In general, simple eukaryotes have less introns than multicellular organisms, but the frequency of introns varies in different lineages (reviewed by Jeffares et al. 2006). In baker's yeast Saccharomyces cerevisiae, only 4% of genes have introns, usually only one per gene (Kupfer et al. 2004). Intron size distribution in yeast, from 50 to 1000 nt, is more narrow than in more complex organisms. 1 Figure 1. Schematic picture of main steps in pre-mRNA processing. Blue boxes, exons; thin black lines, introns; green dots, exon junction complex; thick line, nuclear membrane and pore. RNA pol II" @default.
• W1490999666 created "2016-06-24" @default.
• W1490999666 creator A5083382890 @default.
• W1490999666 date "2010-10-15" @default.
• W1490999666 modified "2023-09-26" @default.
• W1490999666 title "U12-type Spliceosome : Localization and Effects of Splicing Efficiency on Gene Expression" @default.
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