Structure and chromosome localization of the human eosinophil-derived neurotoxin and eosinophil cationic protein genes: Evidence for intronless coding sequences in the ribonuclease gene superfamily

Human genomic DNAs for the eosinophil granule proteins, eosinophil-derived neurotoxin (EDN) and eosinophil cationic protein (ECP), were isolated from genomic libraries. Alignment of EDN (RNS2) and ECP (RNS3) gene sequences demonstrated remarkable nucleotide similarities in noncoding sequences, introns, and flanking regions, as well as in the previously known coding regions. Detailed examination of the 5'-noncoding regions yielded putative TATA and CAAT boxes, as well as similarities to promoter motifs from unrelated genes. A single intron of 230 bases was found in the 5' untranslated region and we suggest that a single intron in this region and an intronless coding region are features common to many members of the RNase gene superfamily. The RNS2 and RNS3 genes were localized to the q24-q31 region of human chromosome 14. It is likely that these two genes arose as a consequence of a gene duplication event that took place approximately 25-40 million years ago and that a subset of anthropoid primates possess both of these genes or closely related genes.     

Isolation of a polyubiquitin promoter and its expression in transgenic potato plants.

A polyubiquitin clone (ubi7) was isolated from a potato (Solanum tuberosum) genomic library using a copy-specific probe from a stress-induced ubiquitin cDNA. The genomic clone contained a 569-bp intron immediately 5' to the initiation codon for the first ubiquitin-coding unit. Two chimeric beta-glucuronidase (GUS) fusion transgenes were introduced into potato. The first contained GUS fused to a 1156-bp promoter fragment containing only 5' flanking and 5' untranslated sequences from ubi7. The second transgene contained GUS translationally fused to the carboxy terminus of the first ubiquitin-coding unit and thus included the intron present in the 5' untranslated region of the polyubiquitin gene. Both ubi7-GUS transgenes were activated by wounding in tuber tissue and in leaves by application of exogenous methyl jasmonate. They were also expressed constitutively in the potato tuber peel (outer 1-2 mm). Both transgenes were actively expressed in mature leaves. Exceptionally high levels of expression were observed in senescent leaves. Transgenic clones containing the ubi7 intron and the first ubiquitin-coding unit showed GUS expression levels at least 10 times higher than clones containing GUS fused to the intronless promoter.     

Intron size, abundance, and distribution within untranslated regions of genes

Most research concerning the evolution of introns has largely considered introns within coding sequences (CDSs), without regard for introns located within untranslated regions (UTRs) of genes. Here, we directly determined intron size, abundance, and distribution in UTRs of genes using full-length cDNA libraries and complete genome sequences for four species, Arabidopsis thaliana, Drosophila melanogaster, human, and mouse. Overall intron occupancy (introns/exon kbp) is lower in 5' UTRs than CDSs, but intron density (intron occupancy in regions containing introns) tends to be higher in 5' UTRs than in CDSs. Introns in 5' UTRs are roughly twice as large as introns in CDSs, and there is a sharp drop in intron size at the 5' UTR-CDS boundary. We propose a mechanistic explanation for the existence of selection for larger intron size in 5' UTRs, and outline several implications of this hypothesis. We found introns to be randomly distributed within 5' UTRs, so long as a minimum required exon size was assumed. Introns in 3' UTRs were much less abundant than in 5' UTRs. Though this was expected for human and mouse that have intron-dependent nonsense-mediated decay (NMD) pathways that discourage the presence of introns within the 3' UTR, it was also true for A. thaliana and D. melanogaster, which may lack intron-dependent NMD. Our findings have several implications for theories of intron evolution and genome evolution in general.     

A full-length and potentially active LINE element is integrated polymorphically within the <Emphasis Type="Italic">IGL</Emphasis> locus in a genomically unstable region of chromosome 22

Leukemic cells of a patient diagnosed with chronic myeloid leukemia (CML) showed a complex BCR-ABL1 rearrangement hidden within a normal appearing karyotype. Previous molecular studies had established that the 3' BCR had recombined at a novel site within the variable region of the immunoglobulin lambda locus ( IGL). A segment of DNA mapping very close to the site of the IGL/3' BCR recombination recognized a previously undescribed insertion polymorphism. A combination of molecular hybridization studies and long-range polymerase chain reaction was used to isolate a 6-kb full-length long interspersed nuclear element (LINE or L1), here designated L1(IGL), which occupies 19% of alleles in the general population. Although unclonable, DNA sequence analysis by a primer walking approach established that L1(IGL) has features characteristic of an actively retrotransposing element. The L1(IGL) element has a 5' untranslated region, two open reading frames (ORF-1 and ORF-2), a 3' untranslated region and terminates in a poly-A tail. We compared the DNA sequence and the predicted amino acid sequence of L1(IGL) with a consensus sequence compiled from seven reported active L1 elements. This analysis indicated that L1(IGL) has high potential for involvement in as yet undetermined somatically and constitutionally acquired disease, not only through recombination mechanisms, but also through retrotransposition events. This full-length L1 element maps close within the IGLlocus to L1.2, one of only nine active L1 elements that have been reported so far. L1(IGL) and L1.2 map within a wider and well-recognized region of genomic instability on chromosome 22.     

Retrotransposed genes such as Frat3 in the mouse Chromosome 7C Prader-Willi syndrome region acquire the imprinted status of their insertion site

Prader-Willi syndrome (PWS) results from loss of function of a 1.0- to 1.5-Mb domain of imprinted, paternally expressed genes in human Chromosome (Chr) 15q11-q13. The loss of imprinted gene expression in the homologous region in mouse Chr 7C leads to a similar neonatal PWS phenotype. Several protein-coding genes in the human PWS region are intronless, possibly arising by retrotransposition. Here we present evidence for continued acquisition of genes by the mouse PWS region during evolution. Bioinformatic analyses identified a BAC containing four genes, Mkrn3, Magel2, Ndn, Frat3, and the Atp5l-ps1 pseudogene, the latter two genes derived from recent L1-mediated retrotransposition. Analyses of eight overlapping BACs indicate that these genes are clustered within 120 kb in two inbred strains, in the order tel–Atp5l-ps1–Frat3–Mkrn3–Magel2–Ndn–cen. Imprinting analyses show that Frat3 is differentially methylated and expressed solely from the paternal allele in a transgenic mouse model of Angelman syndrome, with no expression from the maternal allele in a mouse model of PWS. Loss of Frat3 expression may, therefore, contribute to the phenotype of mouse models of PWS. The identification of five intronless genes in a small genomic interval suggests that this region is prone to retroposition in germ cells or their zygotic and embryonic cell precursors, and that it allows the subsequent functional expression of these foreign sequences. The recent evolutionary acquisition of genes that adopt the same imprint as older, flanking genes indicates that the newly acquired genes become `innocent bystanders' of a primary epigenetic signal causing imprinting in the PWS domain. Received: 22 May, 2001 / Accepted: 16 July 2001     

An element in the 3' untranslated region of human LINE-1 retrotransposon mRNA binds NXF1(TAP) and can function as a nuclear export element

Export of unspliced mRNA to the cytoplasm is required for the replication of all retroviruses. In simian type D retroviruses, the RNA export is mediated by the constitutive transport element (CTE) that binds the cellular nuclear export factor 1, NXF1(TAP). To search for potential cellular RNA substrates for NXF1, we have set up an in vitro selection procedure, using an RNA library expressed from total human genomic DNA. A sequence that was isolated most frequently as independent clones exhibits extensive homology to the 3' untranslated region of expressed LINE1 (L1) retrotransposons. This region, termed L1-NXF1 binding element (L1-NBE) bears no structural resemblance to the viral CTE, but binds NXF1 as strongly as CTE, based on gel mobility shift competition assays. A deletion analysis of the NXF1 protein reveals that CTE and L1-NBE have different, but overlapping, binding domains on NXF1. Placed in an intron, L1-NBE is capable of mediating nuclear export of lariat RNA species in Xenopus laevis oocytes and of an unspliced HIV-1 derived RNA in human 293 cells, suggesting that it may function as a nuclear export element for the intronless L1 mRNA.     

Cloning and sequence comparison of the mouse, human, and chicken engrailed genes reveal potential functional domains and regulatory regions.

We have isolated and characterized genomic DNA clones for the human and chicken homologues of the mouse En-1 and En-2 genes and determined the genomic structure and predicted protein sequences of both En genes in all three species. Comparison of these vertebrate En sequences with the Xenopus En-2 [Hemmati-Brivanlou et al., 1991) and invertebrate engrailed-like genes showed that the two previously identified highly conserved regions within the En protein ]reviewed in Joyner and Hanks, 1991] can be divided into five distinct subregions, designated EH1 to EH5. Sequences 5' and 3' to the predicted coding regions of the vertebrate En genes were also analyzed in an attempt to identify cis-acting DNA sequences important for the regulation of En gene expression. Considerable sequence similarity was found between the mouse and human homologues both within the putative 5' and 3' untranslated as well as 5' flanking regions. Between the mouse and Xenopus En-2 genes, shorter stretches of sequence similarity were found within the 3' untranslated region. The 5' untranslated regions of the mouse, chicken and Xenopus En-2 genes, however, showed no similarly conserved stretches. In a preliminary analysis of the expression pattern of the human En genes, En-2 protein and RNA were detected in the embryonic and adult cerebellum respectively and not in other tissues tested. These patterns are analogous to those seen in other vertebrates. Taken together these results further strengthen the suggestion that En gene function and regulation has been conserved throughout vertebrate evolution and, along with the five highly conserved regions within the En protein, raise an interesting question about the presence of conserved genetic pathways.     

Structure of the gene encoding potato cytosolic pyruvate kinase.

The polymerase chain reaction (PCR) has been used to generate a series of overlapping genomic clones representing 43 bp of 5' untranslated sequence, 63 bp of 3' untranslated sequence and the entire coding sequence of the gene encoding potato cytosolic pyruvate kinase (PKc). This portion of the gene is approximately 4.5 kb in length and is interrupted by three introns, one of which is present in the 5' untranslated region. Southern blot analysis indicates that PKc is encoded by a small gene family, and sequence data from a number of PCR-derived genomic clones indicate that there are as many as six PKc genes. Sequence differences between the PCR-generated genomic clones and a PKc cDNA clone are discussed with respect to the fidelity of Taq polymerase. An alignment of intron placement in the potato PKc gene with intron placement in PK genes from other sources indicates that two of the potato introns correspond to intron positions in other species.