Construction, identification and characterization of a chick elastin cDNA clone

Sheep elastin genes. Isolation and preliminary characterization of a 9.9-kilobase genomic clone

A sheep genomic library containing sheep DNA in the bacteriophage vector Charon 4A was screened for elastin-gene sequences with partially purified, 32P-labelled elastin mRNA (mRNAE). A recombinant containing a 9.9-kb (kilobase) insert was selected from several positive clones by secondary and tertiary screening for further characterization. Positive identification of this elastin clone, designated SE1, was made with radiolabelled mRNAE by hydridization-selected translation and Southern blotting of restriction-enzyme fragments of SE1 DNA. Hybridization of either mRNAE or elastin complementary DNA to restriction fragments of SE1 showed that most of these fragments of SE1 contained elastin-coding sequences. Orientation of the insert was established by preferential hybridization of a short complementary elastin DNA to restriction fragments adjacent to the right arm of Charon 4A. Reciprocal hybridizations of nick-translated SE1 and sheep genomic DNA on Southern blots showed that two restriction fragments of SE1 contained sequence elements which were repeated at high frequency in a restriction-endonuclease-EcoR1 digest of total sheep genomic DNA. In the accompanying paper [Davidson, Shibahara, Boyd, Mason, Tolstoshev & Crystal (1984) Biochem. J. 220, 653-663], it is shown that a subcloned fragment of this elastin gene quantitatively and specifically hybridized to mRNAE sequences in sheep tissue RNA. Electron microscopy of SE1-mRNAE hybrids indicated the presence of at least seven large R-loops. Measurements of these structures indicated that SE1 is likely to contain less than 2 kb of coding sequence and more than 8 kb of intervening sequence, with an average exon size of 120 base-pairs. Thus the elastin gene is distributed over an extended region of the sheep genome and contains numerous intervening and coding sequences.

Characterization of a sheep elastin cDNA clone containing translated sequences

mRNA, isolated from the ligamentum nuchae of fetal sheep by guanidine HCl extraction and oligo(dT) cellulose chromatography, was used to synthesize blunt-ended cDNA molecules by the successive application of AMV reverse transcriptase, DNA polymerase and S1 nuclease. The cDNA was centrifuged on a 15-30% sucrose gradient and molecules greater than 700 bp were tailed with dCTP and cloned into the PstI site of pBR322 which had been tailed with dGTP. Ampicillin-sensitive and tetracycline-resistant colonies were screened by in situ hybridization with elastin-enriched mRNA that had been terminally labeled with 32p. Recombinant plasmids prepared from strongly hybridizing colonies were characterized by restriction mapping and the plasmid with the largest insert (1300 bp) thought to contain elastin sequences was characterized in more detail. The nick-translated cDNA hybridized to a single 3.5 kb mRNA species upon blot hybridization, a size identical to that previously identified for chick elastin mRNA (Burnett et al. (1982) J. Biol. Chem. 259, 1569-1572). Nucleotide sequencing of the 5' end of the cDNA demonstrated a sequence which was extremely GC rich and which corresponded to an amino acid sequence partially homologous to that previously identified in porcine tropoelastin (Foster et al. (1973) J. Biol. Chem. 248, 2876-2879). This is the first report of the identification of a plasmid containing sequences complementary to a translated region of elastin mRNA.

The molecular aspects of elastin gene expression

This report summarizes recent advances in the understanding of the molecular mechanisms which provide the basis for the regulation of elastin synthesis. Methods are available for the direct quantitation of elastin messenger RNA activity and absolute levels of elastin messenger RNA. Current evidence suggests, that at least for some systems, regulation of elastin synthesis primarily occurs at a pretranslational level. Recent advances in the cloning of elastin cDNA and elastin genes will permit the determination of elastin and elastin gene primary structure. These cloned elastin genes will also provide useful probes for the further analysis of the control of elastin gene expression in development and disease states.

Control of elastin synthesis

mRNA was isolated from the thoracic aortas of 16-day chick embryos and used to synthesize blunt-ended heteroduplex molecules consisting of one strand of mRNA and one of cDNA using AMV reverse transcriptase and S1 nuclease. The duplexes were tailed with dCTP and hybridized to the plasmid pBR322 which had been restricted with Pst I and tailed with dGTP. Recombinant plasmids were used to transform E. coli C600 and colonies containing elastin cDNA were selected by in situ hybridization with 32P labeled elastin mRNA and by hybrid selected translation using the nuclease-treated reticulocyte lysate system. mRNA recovered from hybridization to DNA of one clone, pWB1, markedly stimulated incorporation of [3H]valine into a protein which was immunoprecipitable with elastin-specific antibody and had a molecular weight of 72,000, characteristic of tropoelastin. The 230 bp insert of pWB1 was sequenced by the technique of Maxam and Gilbert and found to be derived from a nontranslated region of the 3' end of the mRNA. Nick-translated pWB1 was used to identify and to estimate the relative amounts of elastin mRNA in the developing chick embryo aorta by blot hybridization. A single mRNA species of 3.5 kb hybridized to the pWB1 probe and this species increased greatly in amount between day 7 and day 14. This increase was paralleled by an increase in translatable elastin mRNA and by the rate of elastin synthesis of aortas from various age embryos incubated in vivo. The injection of 150 microgram of hydrocortisone 21-phosphate into 8-day eggs produced a significant increase in both the relative rate of tropoelastin synthesized by the isolated aortas and the relative amount of elastin mRNA. These results suggest that the observed changes in elastin synthesis during development and after hydrocortisone administration are governed by the elastin mRNA content of the aortas.