Disease relevance of Introns

• The intron contains all the conserved features of primary sequence and secondary structure previously described for the group IA introns of eukaryotic organelles and the gram-negative bacteriophage T4 [1].
• Introns in the chicken ovalbumin gene prevent ovalbumin synthesis in E. coli K12 [2].
• Many mitochondria have intron-rich genomes and were probably derived from the alpha subgroup of the purple bacteria (or Proteobacteria), so ancient introns might also have been retained in these bacteria [3].
• Using human immunodeficiency virus-type 1 (HIV-1) proviral DNA and the human CCR5 gene as examples, we show that group II introns can be retargeted to insert efficiently into virtually any target DNA and that the retargeted introns retain activity in human cells [4].
• The most common form of beta-thalassemia among Mediterraneans results from a single nucleotide substitution within the first intervening sequence (IVS-1) of the beta-globin gene [5].

High impact information on Introns

• In response to integrin engagement and surface receptor activation, platelets precisely excise introns from interleukin-1beta pre-mRNA, yielding a mature message that is translated into protein [6].
• The first step in tRNA splicing is the removal of introns catalyzed in yeast by the tRNA splicing endonuclease [7].
• CDY genes retain CDYL exonic sequences but lack its introns [8].
• Intron overaccumulation could lead to spliced exon reopening via a reaction known to be catalyzed by group I introns in vitro [9].
• The telomeric gene is either lacking or interrupted in 226 of 229 patients, and patients retaining this gene (3 of 229) carry either a point mutation (Y272C) or short deletions in the consensus splice sites of introns 6 and 7 [10].

Chemical compound and disease context of Introns

• Fewer branches were found in the poly A(-) RNAs of the nucleus and of late adenovirus transcripts suggesting that excised lariat introns do not accumulate in vivo [11].
• Five short interspersed repetitive elements (SINEs) were found fortuitously in the introns of a steroid hormone receptor AaHR3-2 gene of the yellow fever mosquito, Aedes aegypti, constituting a novel family of tRNA-related SINEs named Feilai [12].
• Translation of RNA by the prokaryotic ribosome, self-splicing of group I introns, HIV replication and hammerhead ribozyme cleavage are inhibited by the aminoglycoside neomycin B [13].
• Expression of malarial SHMT coding sequence (minus the introns) into glyA mutants of Escherichia coli relieved glycine auxotrophy and permitted direct assay of SHMT catalytic activity in bacterial cell lysates [14].

Biological context of Introns

• Exons become correctly ligated, and the excised intron has a lariat structure similar to that of introns from nuclear mRNA [15].
• With the exception of two nucleotide substitutions in the large intervening sequence (intron), the intron and flanking sequences are identical with the nucleotide sequence of the first type determined by Weissmann et al [16].
• Using high-resolution in situ hybridization and computer-aided optical microscope data collection and image analysis, we have determined that the coding portions and introns of the Notch gene, which is not expressed in this tissue, are all contained within the polytene chromosome band 3C7 [17].
• As in the T4 introns, this open reading frame begins in a region that is looped out of the secondary structure, but ends in a highly conserved region of the intron core [1].
• Comparison of the in vitro expression of portions of the ovalbumin gene in nuclei isolated from chronically stimulated oviducts indicates that both structural and intervening sequences are preferentially transcribed in vitro at rates approximately 500 times greater than expected for random transcription of the haploid chick genome [18].

Anatomical context of Introns

• Reverse transcriptase activity associated with maturase-encoding group II introns in yeast mitochondria [19].
• By studying the processing of model introns in transfected plant protoplasts, we have investigated the special requirements for intron recognition by plant cells [20].
• The removal of introns from messenger RNA precursors requires five small nuclear RNAs (snRNAs), contained within ribonucleoprotein particles (snRNPs), which complex with the pre-mRNA and other associated factors to form the spliceosome [21].
• Quantitation of the intron sequences reveals large variations in the amount of both the AChR and actin introns between nuclei within the same myotube, although all nuclei express equivalent amounts of U1 RNA [22].
• A study was made of the function of the intervening sequences in the ovalbumin gene, Radioactively labeled DNA probes for the intervening sequences were prepared and RNA's were isolated from whole cells, nuclei, and polysomes of estrogen-stimulated chick oviducts [23].

Associations of Introns with chemical compounds

• Several mutations next to the branch point and in other parts of the core structure of group II introns are shown to affect lariat formation [15].
• Splicing occurs by the same guanosine-initiated transesterification mechanism characteristic of self-splicing group I introns, but is absolutely dependent upon proteins that are presumably required for correct folding of the pre-rRNA [24].
• The excised introns of pre-mRNAs and intron-containing splicing intermediates are in a lariat configuration in which the 5' end of the intron is linked by a 2'-5' phosphodiester bond (RNA branch) to a single adenosine residue near the 3' end of the intron [25].
• The other is a tyrosine tRNA with a 13 bp intervening sequence located immediately adjacent or very close to the 3' nucleotide of the anticodon [26].
• This structure suggests that introns were not inserted into a previously uninterrupted coding sequence, but instead are products of the evolution of the first pyruvate kinase gene [27].

Gene context of Introns

• Each of the nine introns in the approximately 6.5 kb rbcL locus is approximately 0.5 kb in length [28].
• We have isolated seven alleles of PRP16 that, like the original allele prp16-1, allow splicing of introns with a mutant branch site (UACUAAC to UACUACC), by forming lariat intermediates at the mutant C nucleotide [29].
• Single crossover recombination events between homologous core sequences in the closely linked td and nrdB introns have led to 'exon shuffling [30].
• Transcripts of COXI and COB genes harboring multiple introns are degraded in the absence of PIM1 [31].
• The polypyrimidine tract of mammalian introns is recognized by a 62-kD protein (pPTB) [32].

Analytical, diagnostic and therapeutic context of Introns

• These results are consistent with transcription of the entire ovalbumin gene into a large precursor molecule followed by excision of the intervening sequences and appropriate ligation of the structural sequences to form the mature mRNA [18].
• Restriction mapping and electron microscopic analyses of this cloned DNA have revealed that the structural ovomucoid gene sequences are separated by at least six intervening sequences [33].
• To screen the ts- mutants for pre-mRNA splicing defects, an oligodeoxynucleotide that recognizes one of the introns of the beta-tubulin pre-mRNA was used as a probe in a Northern blot assay to detect accumulation of intron sequences [34].
• The structure and the frequency of the pre-mRNAs of the psbA gene (the gene for the 32-kd protein of photosystem II), which is split by four introns in Euglena chloroplasts was analysed by electron microscopy [35].
• We also have developed a strategy to identify mutations in COL17A1 by use of PCR amplification of genomic DNA, using primers placed on the flanking introns [36].

References