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PubMed1985

The mitochondrial URF1 gene in Neurospora crassa has an intron that contains a novel type of URF.

In Neurospora crassa, a 2670 base-pair segment of the mitochondrial DNA was sequenced including a gene homologous to the mammalian URF1 that was recently shown to encode a subunit of the respiratory chain NADH dehydrogenase complex. URF1 of N. crassa is interrupted by an intron of 1118 base-pairs that divides the protein-coding sequence into two exons of 636 and 480 base-pairs length, respectively. The deduced URF1 polypeptide of 371 residues was aligned with that of other eukaryotes, revealing a degree of conservation similar to that of ubiquitous mitochondrial genes. The two highly conserved stretches coincide with the most polar regions of the otherwise hydrophobic URF1 polypeptides and may constitute functional domains of the complex I subunit. In the exon sequences of URF1, 17 codons occur that are infrequently utilized in other mitochondrial genes of N. crassa, indicating a low translational efficiency or a foreign origin of URF1. The URF1 intron is inserted in the most conserved region. It belongs to group I and contains an open reading frame of 305 codons not continuous with the upstream exon. Sequences convincingly homologous to conserved group I decapeptide motifs were not found in the URF1 intronic unassigned reading frame (URF). However, significant homology was detected to intronic URFs of the respective gene from Podospora anserina, suggesting that these reading frames constitute a novel type of group I intronic URFs. Three species of URF1 transcripts were identified. They arise most probably by subsequent removal of the intron and leader sequences from an URF1 precursor transcript.

Amino Acid Sequence↗
PubMed1981

Beta + thalassemia: aberrant splicing results from a single point mutation in an intron.

We have analyzed the molecular basis of beta + thalassemia by studying the expression of a cloned beta-globin gene in HeLa cells. This beta-globin gene was isolated from a beta + thalassemic patient and differs from the normal beta-globin genes by only a single point mutation within the first intron. The beta + thalassemic and the normal beta-globin genes were cloned into an SV40-pBR328 vector and introduced into HeLa cells by calcium phosphate coprecipitation. We assayed the RNA from these transfected HeLa cells by S1 nuclease mapping and cDNA sequencing to detect the nature of the defect in beta-globin gene expression. While the transcripts of the normal beta-globin gene are processed correctly, the first intron of the beta + thalassemic beta-globin gene is incorrectly spliced in about 90% of the mRNA because of an additional 3; splice site that has been created by the point mutation. This incorrectly spliced mRNA is effectively exported to the cytoplasm, where it would conceivably be translated to give a truncated globin chain of 35 amino acids. The remaining 10% of the mRNA transcribed from the beta+ thalassemic globin gene is correctly spliced and can therefore be translated to give normal beta-globin. In addition to the incorrect splicing of the first intron, the splicing of both introns is retarded, which results in the accumulation of unspliced pre-mRNA. This suggests that removal of the first intron might facilitate splicing of the second intron.

Base Sequence↗
PubMed1983

An mRNA maturase is encoded by the first intron of the mitochondrial gene for the subunit I of cytochrome oxidase in S. cerevisiae.

We have localized ten oxi3- mutations in the first, al1, intron of the coxl gene. All are splicing deficient, being unable to excise the intron. Complementation experiments disclose several domains in the intron al1: the 5'-proximal and 3'-proximal domains harbor cis-dominant mutations, while trans-recessive ones are located in the intron's open reading frame. Comprehensive analyses of allele-specific polypeptides accumulating in mutants show that they result from the translation of the intron's ORF. We conclude that a specific mRNA maturase involved in splicing of oxidase mRNA is encoded by the intron al1 in a manner similar to the cytochrome b mRNA maturase.

Chromosome Mapping↗
PubMed1986

Excised group II introns in yeast mitochondria are lariats and can be formed by self-splicing in vitro.

Excised group II introns in yeast mitochondria appear as covalently closed circles under the electron microscope. We show that these circular molecules are branched and resemble the lariats arising through splicing of nuclear pre-mRNAs in yeast and higher eukaryotes. One member of this intron class (aI5c in the gene for cytochrome c oxidase subunit I) is capable of self-splicing in vitro, giving correct exon-exon ligation and resulting in the appearance of both linear and lariat forms of the excised intron. Nuclease digestion of the latter molecules reveals the presence of a complex oligonucleotide with the probable structure AGU, which thus resembles the branch point formed in the spliceosome-dependent reactions undergone by nuclear pre-mRNAs. Unlike group I introns, this group II intron is not demonstrably dependent on GTP for self-splicing and circularization of the isolated, linear intron is not observed. A model accounting for these observations is presented.

Base Sequence↗
PubMed1985

Molecular consequences of specific intron mutations on yeast mRNA splicing in vivo and in vitro.

We have altered the TACTAAC sequence in the yeast CYH2m gene intron to TACTACC. This mutation changes the nucleotide at the normal position of the branch in intron RNA lariats produced during pre-mRNA splicing, and it prevents splicing in vivo. In a yeast pre-mRNA splicing system, CYH2m pre-mRNA carrying the TACTACC mutation is not specifically cut or rearranged in any way. Substitution of an A for the first G of the CYH2m intron, converting the highly conserved GTATGT 5' splice site sequence to ATATGT, also blocks intron excision in vivo and in vitro: pre-mRNA carrying this mutation was still cut normally at the mutant 5' splice site in vitro, to give authentic exon 1 and an intron-exon 2 lariat RNA with an A-A 2'-5' phosphodiester linkage at the branch point. This lariat RNA is a dead-end product. The subsequent cleavage at the 3' splice site is therefore sensitive to the sequence of the 5' end of the intron attached at the branch point.

Base Sequence↗
PubMedDate not supplied

A test for intron function in the yeast actin gene.

Many eukaryotic genes contain intervening sequences (IVS), but the rationale for their existence remains a mystery. Previous studies done in our laboratory demonstrated that the intron in a yeast tRNATyr gene, SUP6, does have a function. We used the same approach to determine the role of introns in nuclear genes encoding messenger RNAs. A single actin gene with one intron exists in Saccharomyces cerevisiae. The level of actin in yeast appears to be crucial to viability: either too much or too little actin inhibits growth. Therefore, small effects on synthesis of actin protein resulting from the removal of the actin gene intron would be expected to cause measurable changes in cell growth. In the present study, an intron-deleted actin gene was constructed in vitro and was used to replace the single resident actin gene in a haploid strain. Analysis of the cells carrying the intron-deleted actin gene shows that the intervening sequence is not essential for actin gene expression.

Actins↗
PubMed1982

Relationship between the total size of exons and introns in protein-coding genes of higher eukaryotes.

We have attempted to ascertain the correlation between the genetic information content in the exons and the surrounding intron sequences with regard to their spatial arrangement within a gene. A comparison is made of the sizes, taken from recent publications, of exons and introns of approximately equal to 80 different protein-coding chromosomal genes, mostly from higher eukaryotes. The exons of these genes do not show very marked variation in size and can be classified into three major discrete and two minor additional size groups, whereas individual introns vary considerably in size within and between genes. Notwithstanding, the overall length of all introns present within a given gene is a function of the total size, mostly corresponding to the total genetic information content, of the exons. Three cases that violate this exon-size dependency of introns are genes coding for (i) histone H1, feather keratin, and interferons, (ii) tubulin and actin, and (iii) silk fibroin. The exons of these genes are larger than 0.7 kilobase pair in total size and the genes show a strong sequence homogeneity among the repetitious family members or internal repeats of coding sequences within the gene. We propose that conservation of sequences, which is required by the family members, internal repeats, or the entire gene, would actually motivate the removal of introns.

Animals↗
PubMed1985

Molecular structure of the human cytoplasmic beta-actin gene: interspecies homology of sequences in the introns.

A recombinant phage that carries the cytoplasmic beta-actin gene was isolated from a human DNA library. The nucleotide sequence of this gene was determined. The amino acid sequence deduced from the nucleotide sequence matches perfectly that of beta-actin from human fibroblasts. The gene contains five introns. A large intron was found in the 5' untranslated region six nucleotides upstream from the ATG initiation codon. Four introns were found within the coding region at codons specifying amino acids 41/42, 121/122, 267, and 327/328. In contrast to the human cardiac muscle actin gene, the aorta-type smooth muscle actin gene, and the stomach-type smooth muscle actin gene, the beta-actin gene lacks the codon for cysteine between the ATG initiation codon and the codon for the NH2-terminal amino acid of the mature protein. Hybridization of genomic DNA with DNA fragments derived from intron I in the 5' untranslated region and from intron III strongly suggests the presence of a single beta-actin gene in the human genome. The DNA sequences of the coding region, of the 3' untranslated region, and of the sequence block between the "CCAAT" box and "TATA" box in the 5' flanking DNA of the human beta-actin gene are highly homologous to the corresponding sequences of the rat and chicken beta-actin genes. Unexpectedly, the sequence of intron III of the human beta-actin gene shows considerable homology to that of the rat beta-actin gene.

Actins↗
PubMed2000

A diverse population of introns in the nuclear ribosomal genes of ericoid mycorrhizal fungi includes elements with sequence similarity to endonuclease-coding genes.

Ericoid mycorrhizal fungi form symbioses with the roots of members of the Ericales. Although only two genera have been identified in culture, the taxonomic diversity of ericoid symbionts is certainly wider. Genetic variation among 40 ericoid fungal isolates was investigated in this study. PCR amplification of the nuclear small-subunit ribosomal DNA (SSU rDNA) and of the internal transcribed spacer (ITS), followed by sequencing, led to the discovery of DNA insertions of various sizes in the SSU rDNA of most isolates. They reached sizes of almost 1,800 bp and occurred in up to five different insertion sites. Their positions and sizes were generally correlated with morphological and ITS-RFLP grouping of the isolates, although some insertions were found to be optional among isolates of the same species, and insertions were not always present in all SSU rDNA repeats within an isolate. Most insertions were identified as typical group I introns, possessing the conserved motifs characteristic of this group. However, other insertions lack these motifs and form a distinct group that includes other fungal ribosomal introns. Alignments with almost 70 additional sequences from fungal nuclear SSU rDNA introns indicate that introns inserted at the same site along the rDNA gene are generally homologous, but they also suggest the possibility of some horizontal transfers. Two of the ericoid fungal introns showed strong homology with a conserved motif found in endonuclease genes from nuclear rDNA introns.

Amino Acid Sequence↗
PubMed2005

Ultraconserved elements in insect genomes: a highly conserved intronic sequence implicated in the control of homothorax mRNA splicing.

Recently, we identified a large number of ultraconserved (uc) sequences in noncoding regions of human, mouse, and rat genomes that appear to be essential for vertebrate and amniote ontogeny. Here, we used similar methods to identify ultraconserved genomic regions between the insect species Drosophila melanogaster and Drosophila pseudoobscura, as well as the more distantly related Anopheles gambiae. As with vertebrates, ultraconserved sequences in insects appear to occur primarily in intergenic and intronic sequences, and at intron-exon junctions. The sequences are significantly associated with genes encoding developmental regulators and transcription factors, but are less frequent and are smaller in size than in vertebrates. The longest identical, nongapped orthologous match between the three genomes was found within the homothorax (hth) gene. This sequence spans an internal exon-intron junction, with the majority located within the intron, and is predicted to form a highly stable stem-loop RNA structure. Real-time quantitative PCR analysis of different hth splice isoforms and Northern blotting showed that the conserved element is associated with a high incidence of intron retention in hth pre-mRNA, suggesting that the conserved intronic element is critically important in the post-transcriptional regulation of hth expression in Diptera.

Animals↗
PubMed1999

The AG dinucleotide terminating introns is important but not always required for pre-mRNA splicing in the maize endosperm

Previous RNA analysis of lesions within the 15 intron-containing Sh2 (shrunken2) gene of maize (Zea mays) revealed that the majority of these mutants affect RNA splicing. Here we decipher further two of these mutants, sh2-i (shrunken2 intermediate phenotype) and sh2-7460. Each harbors a G-to-A transition in the terminal nucleotide of an intron, hence destroying the invariant AG found at the terminus of virtually all nuclear introns. Consequences of the mutations, however, differ dramatically. In sh2-i the mutant site is recognized as an authentic splice site in approximately 10% of the primary transcripts processed in the maize endosperm. The other transcripts exhibited exon skipping and lacked exon 3. A G-to-A transition in the terminus of an intron was also found in the mutant sh2-7460, in this case intron 12. The lesion activates a cryptic acceptor site downstream 22 bp within exon 13. In addition, approximately 50% of sh2-7460 transcripts contain intron 2 and 3 sequences.

Journal Article↗
PubMed1984

Molecular basis of the 'box effect', A maturase deficiency leading to the absence of splicing of two introns located in two split genes of yeast mitochondrial DNA.

In the mitochondrial DNA of Saccharomyces cerevisiae, the genes cob-box and oxi3, coding for apocytochrome b and cytochrome oxidase subunit I respectively, are split. Several mutations located in the introns of the cob-box gene prevent the synthesis of cytochrome b and cytochrome oxidase subunit I (this is known as the 'box effect').-We have elucidated the molecular basis of this phenomenon: these mutants are unable to excise the fourth intron of oxi3 from the cytochrome oxidase subunit I pre-mRNA; the absence of a functional bI4 mRNA maturase, a trans-acting factor encoded by the fourth intron of the cob-box gene explains this phenomenon. This maturase was already known to control the excision of the bI4 intron; consequently we have demonstrated that it is necessary for the processing of two introns located in two different genes. Mutations altering this maturase can be corrected, but only partially, by extragenic suppressors located in the mitochondrial (mim2) or in the nuclear (NAM2) genome. The gene product of these two suppressors should, therefore, control (directly or indirectly) the excision of the two introns as the bI4 mRNA maturase normally does.

Base Sequence↗
PubMed1984

Yeast mitochondria contain a linear RNA strand complementary to the circular intronic bI1 RNA of cytochrome b.

bI1 RNA (excised from the first intron of the long form of the cytochrome b gene of Saccharomyces cerevisiae mitochondria) hybridizes with the two strands of a Bg/II-MboI DNA segment from this region. This fraction is resistant to digestions by DNase I and RNase T1 and disappears completely upon alkali hydrolysis. Strand-specific labeling of an intronic DNA fragment, cloned in pBR322 plasmid, was accomplished through the use of a T4 DNA polymerase. The purity of the probes was demonstrated by cloning an exon-intron fragment and labeling it by the same procedure; mRNA and pre-mRNA bands hybridized only with the transcribed DNA strand whereas bI1 RNA hybridized with the two strands under the stringent washing conditions employed (tm + 20 degrees C). Several experimental results argue against the possibility that the observation of two complementary bI1 RNA strands results from a partial self-complementarity of the RNA. A pre-mRNA intermediate from a box8 (G5046) mutant, still containing this intron, hybridizes only with the transcribed DNA strand of the pure intronic probe. The amount of the non-sense bI1 RNA strand is very low, in cells from two wild-type strains, relative to the sense RNA strand during the early stages of growth on glucose. It increases as the cells are released from glucose repression. bI1 RNA is resistant to RNase. Very little self-complementarity is seen by computer analysis of the sequence. Purified bI1 RNA is seen by electron microscopy under non-denaturing conditions as a mixture of double-stranded circular and linear molecules thus confirming the existence of the two complementary strands. The disappearance of all material following alkali hydrolysis demonstrates that these are indeed two RNA strands. Under fully denaturing conditions a mixture of single-stranded circular and linear molecules is seen as reported previously (Cell, 19, 321-329, 1980). We conclude that yeast mitochondria contain the two complementary bI1 RNA strands, one circular and the other linear. Considering a largely asymmetrical transcription of the mitochondrial genome in yeast and assuming that circularization of some intronic RNAs is part of RNA processing, we do not believe that the two strands are each a mixture of linear and circular molecules. The ratio of non-sense to sense bI1 RNA in a cytoplasmic petite mutant, A1B1, also varies according to growth conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Base Sequence↗
PubMed1984

Role of intron-contained sequences in formation of moloney murine leukemia virus env mRNA.

Formation of the Moloney murine leukemia virus envelope mRNA involves the removal of a 5,185-base pair-long intron. Deletion analysis of two Moloney murine leukemia virus-derived expression vectors revealed the existence of two short regions within the viral intron which are required for the efficient formation of the spliced RNA species. One region was present upstream from the 3' splice junction, extended at least 85 nucleotides beyond the splice site, and was not more than 165 nucleotides long. As yeast polymerase II introns, the Moloney murine leukemia virus intron contains the sequence 5'-TACTAAC-3' 15 nucleotides upstream from the 3' splice site. A second region located in the middle of the intron, within a 560-nucleotide-long sequence, was also essential for formation of the spliced RNA species. The efficient splicing of the env mRNA in the absence of expression of viral genes raises the possibility that similar mechanisms are used to remove introns of (some) cellular genes.

Base Sequence↗
PubMed2001

Evolution of the FAD2-1 fatty acid desaturase 5' UTR intron and the molecular systematics of Gossypium (Malvaceae).

The FAD2-1 microsomal omega-6 desaturase gene contains a large intron ( approximately 1133 bp [base pairs]) in the 5' untranslated region that may participate in gene regulation and, in GOSSYPIUM:, is evolving at an evolutionary rate useful for elucidating recently diverged lineages. FAD2-1 is single copy in diploid GOSSYPIUM: species, and two orthologs are present in the allotetraploid species. Among the diploid species, the D-genome FAD2-1 introns have accumulated substitutions 1.4-1.8 times faster than the A-genome introns. In the tetraploids, the difference between the D-subgenome introns and their A-subgenome orthologs is even greater. The substitution rate of the intron in the D-genome diploid G. gossypioides more closely approximates that of the A genome than other D genome species, highlighting its unique evolutionary history. However, phylogenetic analyses support G. raimondii as the closest living relative of the D-subgenome donor. The Australian K-genome species diverged 8-16 million years ago into two clades. One clade comprises the sporadically distributed, erect to suberect coastal species; a second clade comprises the more widely spread, prostrate, inland species. A comparison of published gene trees to the FAD2-1 intron topology suggests that G. bickii arose from an early divergence, but that it carries a G. australe-like rDNA captured via a previously undetected hybridization event.

Journal Article↗
PubMed1985

Intron positions are conserved in the 5' end region of myosin heavy-chain genes.

We have determined the 5' end sequence of the rat embryonic skeletal muscle myosin heavy-chain (MHC) gene comprising the first three amino-terminal coding exons. Comparison with the corresponding regions of the rat ventricular alpha and the nematode Caenorhabditis elegans unc-54 MHC genes (Mahdavi, V., Chambers, A.P., and Nadal-Ginard, B. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 2626-2630; Karn, J., Brenner, S., and Barnett, L. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 4253-4257) shows that the degree of amino acid sequence conservation increases from the first to the third exon. Intron positions between these exons are maintained in all three genes studied, whereas size and sequence of corresponding introns are highly divergent. In contrast to the rat MHC genes where the coding region is highly split throughout its entire length, only the 5' end region is frequently interrupted by introns in the nematode gene indicating the potential importance of these introns in gene structure and expression. The occurrence of "preferential" intron positions in the MHC genes suggests the existence of a highly split ancestral MHC gene from which different evolutionary lineages removed and/or added specific sets of introns.

Amino Acid Sequence↗
PubMed1986

Phase II study of rDNA alpha-2 interferon (INTRON A) in patients with multiple myeloma utilizing an escalating induction phase.

Twenty previously treated patients with multiple myeloma were treated with rDNA human alpha-2 interferon (INTRON A) in a phase II trial. Patients received an induction phase of therapy consisting of 3-100 X 10(6) IU/m2 iv given every other day pending myelosuppression. Patients then received 10 X 10(6) IU/m2 three times a week sc. In patients not responding to the iv and sc protocol, prednisone (20 mg orally) was given with each dose of INTRON A to determine whether additional responses could be produced and whether toxicity could be reduced. During the sc phases of therapy, INTRON A was escalated pending hematologic and nonhematologic toxicity. Three partial remissions were achieved in patients receiving the initial iv/sc therapy, and one additional patient responded when prednisone was added (durations of remission, 5, 6, 8, and 9 months). Myelosuppression was the dose-limiting toxic effect in both the iv and sc phases of therapy. Constitutional symptoms (flu-like) were seen in the majority of patients, but were tolerable. With the utilization of prednisone, flu-like symptoms were reduced in frequency and degree. Escalation of the dose of INTRON A was possible in the majority of patients when prednisone was added; however, only one patient (of seven) responded to combination therapy. INTRON A can produce remissions in 20% of patients with previously treated multiple myeloma. No improvement in the response rate was achieved utilizing a high-dose induction program. Although the dose of INTRON A could be escalated when prednisone was added, the response rate was not enhanced.

Adult↗
PubMed1983

Genes encoding actin in higher plants: intron positions are highly conserved but the coding sequences are not.

We have isolated actin genes from genomic libraries of two highly diverged plants, maize and soybean. The complete nucleotide sequences of a maize actin gene, MAc1, and a soybean actin gene, SAc1, were determined. The nucleotide sequences of these two actin genes and of a previously sequenced soybean actin gene were compared with the actin gene sequences from a wide spectrum of evolutionarily diverged eukaryotes. Some striking features pertinent to the evolution and function of the plant actin gene families have emerged. The deduced amino acid sequence of the plant actins resembles both cytoplasmic- and muscle-specific actins. DNA sequence analysis as well as genomic blotting experiments using cloned actin sequences as probes show that large sequence heterogeneity exists among members of the plant actin multigene families and between genes from two highly diverged plant species. The sequences of the first nine amino acids at the amino terminal end of the plant actins are far more conserved between distant plant actins than the corresponding sequences in distantly related animal actin genes, suggesting a unique and conserved function for the NH2 terminal sequence in higher plants. The soybean and maize actin genes examined each contain three introns in precisely the same positions, quite contrary to the divergent placement of introns observed in animal, protozoan, and fungal actins. The position of the first intron in soybean and maize actin genes corresponds precisely to the position of an intron found in a nematode actin gene. The position of the second intron coincides with one found in rat and chicken skeletal actin genes. These data suggest that the numerous introns found in all actins are of ancient origin. The degree of silent substitution and replacement substitution was compared among plant actin genes and to those of animal, protozoan, and yeast actin genes. It is clear that the silent substitution sites are saturated among all the genes compared, whereas the replacement sites have diverged in only 5-17% of their possible positions. By these criteria the most distant animal actins are only 6% diverged. The three plant actin genes examined are 8-10% diverged in replacement sites from each other and approximately 14% diverged in replacement sites from any of the animal actins examined. The data in this manuscript suggest that the families of soybean and maize actin genes may have diverged from a single common ancestral actin gene long before the divergence of monocots and dicots.

Actins↗