The cDNA coding for xanthine dehydrogenase (XD) is isolated from mouse liver mRNA by cross-hybridization with a DNA fragment of the Drosophila melanogaster homologue. Two lambda bacteriophage overlapping clones represent the copy of a 4538-nucleotide-residue-long transcript with an open reading frame of 4005 nucleotide residues, coding for a putative polypeptide of 1335 amino acid residues. Comparison of the deduced amino acid sequence of the mouse XD with those of the Drosophila and the rat homologues shows a high conservation of this protein (55% identity between mouse and Drosophila, and 94% identity between mouse and rat). RNA blotting analysis demonstrates that interferon-alpha (IFN-alpha) and its inducers, i.e. poly(I).poly(C), bacterial lipopolysaccharide (LPS) and tilorone (2,7-bis-[2-(diethylamino)ethoxy]fluoren-9-one), increase the expression of XD mRNA in liver. Poly(I).poly(C) also induces XD mRNA in several other tissues in vivo. Protein synthesis de novo is not required for the elevation of XD mRNA after IFN-alpha treatment, since cycloheximide does not block the induction. The elevation of XD mRNA concentration is relatively fast and precedes the induction of both XD and xanthine oxidase (XO) enzymic activities.
We have hybridized a human DNA fraction corresponding to the GC-richest and gene-richest isochore family, H3, on compositional fractions of DNAs from 12 mammalian species and three avian species, representing eight and three orders, respectively. Under conditions in which repetitive sequences are competed out, the H3 isochore probe only or predominantly hybridized on the GC-richest fractions of main-band DNA from all the species investigated. These results indicate that single-copy sequences from the human H3 isochores share homology with sequences located in the compositionally corresponding compartments of the vertebrate genomes tested. These sequences are likely to be essentially formed by conserved coding sequences. The present results add to other lines of evidence indicating that isochore patterns are highly conserved in warm-blooded vertebrate genomes. Moreover, they refine recent reports (Sabeur et al., 1993; Kadi et al., 1993), and correct them in some details and also in demonstrating that the shrew genome does not exhibit the general mammalian pattern, but a special pattern.
The mouse genome is a mosaic of isochores, consisting of long (> 300 kb), compositionally homogeneous DNA segments that can be divided into two GC-poor families, L1 and L2, representing 56% of the genome, and two GC-rich families, H1 and H2, representing 26% and 7% of the genome, respectively, the remaining 11% being formed by satellite and ribosomal DNAs. (GC is the molar fraction of guanine + cytosine in DNA.) The mouse genome differs from the human genome (which is representative of most mammalian genomes) because it shows a narrower compositional spectrum of isochores and it has a karyotype formed exclusively by acrocentric chromosomes. The chromosomal distribution of the four isochore families, as investigated here by in situ hybridization of single-copy sequences from compositional DNA fractions, has shown that G(lemsa) bands are essentially composed of GC-poor isochores, whereas R(everse) bands comprise three subsets of bands: R' bands, containing GC-poor isochores and GC-rich isochores of the H1 family, and T and T' bands, containing all H2 isochores (in addition to other isochores), the former containing a higher proportion of H2 isochores than the latter. Mouse T and T' bands are generally syntenic with, and are compositionally related to, human T and T' bands and have the highest gene concentrations. These findings indicate that the distribution of isochore families and genes in chromosomal bands is basically similar in mouse and in human genomes, in spite of their remarkable differences and their extremely large phylogenetic distance.
We have hybridized the vertebrate telomeric sequence (TTAGGG)n on DNA compositional fractions from 13 mammalian species and 3 avian species, representing 9 and 3 orders, respectively. Our results indicate that the 50- to 100-kb fragments derived from telomeric regions are composed of GC-rich and GC-richest isochores. Previous works from our laboratory demonstrated that single-copy sequences from the human H3 isochore family (the GC-richest and gene-richest isochore in the human genome) share homology with compositionally correlated compartments of warm-blooded vertebrates. This correlation suggested that the GC-richest isochores are, as in the human genome, the gene-richest regions of warm-blooded vertebrates' genome. Moreover, this evidence suggests that telomeric regions are the most gene-dense region of all warm-blooded vertebrates. The implications of these findings are discussed.
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