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1.
We previously found that the splicing of exon 5 to exon 6 in the rat beta-TM gene required that exon 6 first be joined to the downstream common exon 8 (Helfman et al., Genes and Dev. 2, 1627-1638, 1988). Pre-mRNAs containing exon 5, intron 5 and exon 6 are not normally spliced in vitro. We have carried out a mutational analysis to determine which sequences in the pre-mRNA contribute to the inability of this precursor to be spliced in vitro. We found that mutations in two regions of the pre-mRNA led to activation of the 3'-splice site of exon 6, without first joining exon 6 to exon 8. First, introduction of a nine nucleotide poly U tract upstream of the 3'-splice site of exon 6 results in the splicing of exon 5 to exon 6 with as little as 35 nucleotides of exon 6. Second, introduction of a consensus 5'-splice site in exon 6 led to splicing of exon 5 to exon 6. Thus, three distinct elements can act independently to activate the use of the 3'-splice site of exon 6: (1) the sequences contained within exon 8 when joined to exon 6, (2) a poly U tract in intron 5, and (3) a consensus 5'-splice site in exon 6. Using biochemical assays, we have determined that these sequence elements interact with distinct cellular factors for 3'-splice site utilization. Although HeLa cell nuclear extracts were able to splice all three types of pre-mRNAs mentioned above, a cytoplasmic S100 fraction supplemented with SR proteins was unable to efficiently splice exon 5 to exon 6 using precursors in which exon 6 was joined to exon 8. We also studied how these elements contribute to alternative splice site selection using precursors containing the mutually exclusive, alternatively spliced cassette comprised of exons 5 through 8. Introduction of the poly U tract upstream of exon 6, and changing the 5'-splice site of exon 6 to a consensus sequence, either alone or in combination, facilitated the use of exon 6 in vitro, such that exon 6 was spliced more efficiently to exon 8. These data show that intron sequences upstream of an exon can contribute to the use of the downstream 5'-splice, and that sequences surrounding exon 6 can contribute to tissue-specific alternative splice site selection.  相似文献   

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Concerning the signals which direct excision of introns from mRNA precursors in higher eukaryotic genes, consensus 9-nucleotide sequence, (CA)AG/GT(AG)AGT, has been proposed with the 5'-splice site, but actual 5'-splice site sequences differ from it in a greater or lesser degree. We analyzed 5'-splice site sequence of human beta-globin gene by quantification method (categorical discriminant analysis) proposed previously. Analysis of 13-nucleotide sequences and deleted sequences showed that 9-nucleotide sequences in the consensus region are almost sufficient to define 5'-splice signal. To confirm this view, we examined a number of beta-globin mutant genes, where nucleotide changes occur at the authentic 5'-splice site of the first intron and cause beta-thalassemia phenotype. Our method could explain why such mutations abolish the 5'-splice site and cryptic 5'-splice sites are activated.  相似文献   

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Strength of splice signal sequence plays an important role in mammalian pre-mRNA splicings. In the splicing of human beta-globin thalassemia pre-mRNA, a 25-nucleotide deletion covering the signal sequence at 3'-splice site of intron 1 causes unsplicing of intron 1, while splicing of intron 2 occurs normally. This gives abnormal mRNA and beta-thalassemia disease. If 3'-splice site of intron 1 is inactivated, two 5'-splice signals of introns 1 and 2 compete with each other for the 3'-splice site of intron 2. Our quantification analysis revealed that the 5'-splice signal of intron 2 is stronger than that of intron 1, explaining the mechanism for unsplicing of intron 1.  相似文献   

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With regard to the signals that direct excision of introns from mRNA precursors in higher eukaryote genes, a consensus sequence, (sequence; see text); has been proposed for the 3'-splice site, but actual 3'-splice site sequences differ from it to a greater or lesser degree. In the present paper, nucleotide sequences were transformed into categorical data, and quantification analysis (class II), as proposed by Hayashi, was applied to the system. Categorical weights given to variables related to position and the species of nucleotide were estimated so that the two classes of 3'-splice site sequences and sequences other than 3'-splice site might be discriminated most distinctly. The 3'-splice site signals were then characterized in terms of these categorical weight values. We also calculated partial correlation coefficient values, which explain the relative importance of each position in the 3'-splice site signal sequence.  相似文献   

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Nucleotide sequence of the exon-intron junction in human alpha-globin gene was analyzed by quantification method proposed previously. Using sample score of 9-nucleotide sequence at 5'-splice site, we examined strength of the splice signal. We further studied a mutant of alpha-thalassemia, where pentanucleotide deletion occurs around 5'-splice junction of the first intron. This mutation abolishes the normal 5'-splice site completely, but activates a cryptic site lying in the first exon. Such a behaviour was well explained in terms of our sample scoring scheme.  相似文献   

10.
Alternative pre-mRNA splicing may be the most efficient and widespread mechanism to generate multiple protein isoforms from single genes. Here, we describe the genomic analysis of one of the most frequent types of alternative pre-mRNA splicing, alternative 5'- and 3'-splice-site selection. Using an EST-based alternative splicing database recording >47,000 alternative splicing events, we determined the frequency and location of alternative 5'- and 3'-splice sites within the human genome. The most common alternative splice sites used in the human genome are located within 6 nucleotides (nt) of the dominant splice site. We show that the EST database overrepresents alternative splicing events that maintain the reading frame, thus supporting the concept that RNA quality-control steps ensure that mRNAs that encode for potentially harmful protein products are destroyed and do not serve as templates for translation. The most frequent location for alternative 5'-splice sites is 4 nt upstream or downstream from the dominant splice site. Sequence analysis suggests that this preference is a consequence of the U1 snRNP binding sequence at the 5'-splice site, which frequently contains a GU dinucleotide 4 nt downstream from the dominant splice site. Surprisingly, approximately 50% of duplicated 3'-YAG splice junctions are subject to alternative splicing. This high probability of alternative 3'-splice-site activation in close proximity of the dominant 3'-splice site suggests that the second step of the splicing may be prone to violate splicing fidelity.  相似文献   

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