Isolation of feather keratin mRNA and its translation in a rabbit reticulocyte cell-free system

Differential post-translational modification of human type I keratins synthesized in a rabbit reticulocyte cell-free system

The translation products synthesized in a rabbit reticulocyte lysate system, from total cellular mRNA from the human epithelial cell-line ME-180, have been examined. Keratin proteins are prominent among these translation products, and they precisely coelectrophorese in sodium dodecyl sulfate-polyacrylamide gels with keratins purified from the cells. Type-I, acidic, keratins which are acetylated in vivo, are also acetylated by the reticulocyte lysate. Examination by two-dimensional electrophoresis, of two acidic keratins known to be phosphorylated in vivo reveals that only one of these proteins is phosphorylated in the lysate system. Phosphorylation of this protein occurs after release of the completed polypeptide chain from the ribosome. The protein phosphorylated by the lysate is known to be the only ME-180 phosphokeratin modulated by cyclic AMP, reflecting in vitro the differential modification of ME-180 keratins in vivo.

Organisation of feather keratin genes in the chick genome

A genomic clone containing sequences of five feather keratin genes has been isolated using cDNA to chicken embryonic feather keratin mRNA as a probe. The clone probably represents part of a longer cluster of tandemly spaced genes; the genes are evenly spaced with a centre to centre separations of 3.3 kb and are transcribed from the same DNA strand, suggesting that the cluster has arisen by a series of tandem duplications. The organisation and complete sequence of the central gene has been determined. The protein encoded by the gene contains 97 amino acids and its sequence is typical of proteins of the embryonic and adult feather family. The transcript from the gene has been deduced to contain a long 3' non-coding region of 435 nucleotides and a 58 base 5' non-coding region interrupted in the gene 21 bases prior to the initiation codon by the gene's only intron of 324 bases.

Chemical synthesis of the hexanucleotide d(A-C-C-A-G-C) required to isolate fibroin mRNA on an affinity column

The synthesis of the hexanucleotide d)A-C-C-A-G-C), complementary to the 2 major triplets of fibroin mRNA, using the phosphotriester methodology is described. The protected dinucleotides ((MeO)2Tr)dbzA.anC, ((MeO)2Tr)danC.bzA and ((meO)2Tr)dacG.anC were synthesized; the latter two were detritylated and joined in stepwize fashion to the 1st to form the protected hexanucleotide ((MeO)2Tr)dbzA.anC.anC.bzA.acG.anC. The latter was deblocked with NH3 and acid to form the hexanucleotide d(A-C-C-A-G-C). In view of the ability of a prototype affinity column, oligo dC-cellulose, to isolate fibroin mRNA, prospects appear excellent for the d(A-C-C-A-G-C)-cellulose affinity column isolation of fibroin mRNA.

[RNA metabolism in chick embryo skin]

Explants from 7, 8, 9, 11, 13-day chick embryonic skin incorporating (3H) Uridine for different periods 1 hr, 3 or 4 hr and a chase with actinomycin) are studied with respect to free (F) or membrane bound (B) cytoplasmic polysomes and to RNA extracted from them. Polysome specific activity decreases at older stages but the amount of polysomes increases due to increased protein synthesis. At each stage B polysomes are less abundant but more radioactive than F polysomes. RNA extracted from each kind is analysed on sucrose gradients: one half of each fraction is precipitated by TCA to estimate total radioactivity, the other is retained on millipore at high salt concentration to estimate radioactivity in messenger-like RNAs due to their poly-A sequences. The pattern of the labelling of the different fractions of RNA changes with the length of incorporation, the stages of explants and the kind of polysomes (F or B); at 11-13 days the incorporation is slow, radioactivity is low and distributed among several peaks of poly-A RNA; at 7-8 days the incorportion is rapid, dispersed throughout the gradient; at 9 days, a midway stage, incorporation is particularly high into 12S and 24S fractions from B RNA. In the 5 studied stages the labelling of this 12S occurs early, remains for a longer time and cannot be chased. These observations suggest stability of the 12S RNA. Since, in 14-day chick embryos, feather keratin m RNA has been shown to sediment at 12S and although our experiments have been done with total skin because this differentiating tissue is the site of extensive interactions between dermis and epidermis, they suggest that this 12S RNA is the actual keratin m RNA and might be synthesised some days before the onset of keratin synthesis. Its template ability will be investigated at earlier stages.

Cleavage of adenovirus messenger RNA and of 28S and 18S ribosomal RNA by RNase III

Escherichia coli ribonuclease III cleaves adenovirus messenger RNA and mammalian 28S and 18S ribosomal RNA. Fragmentation is not random, but in each case a specific collection of products is generated. This points to the potential use of the enzyme as a tool for specific fragmentation of RNA. Cleavage by RNase III abolishes the capability of adenovirus messenger RNA to direct cell-free synthesis of virus polypeptides.

Inhibitor of protein synthesis co-isolating with polyribosomal RNA

Although low concentrations of total polyribosomal RNA from porcine parotid glands or rat pituitary cells in culture (GH3) isolated by standard dodecylsulphate/phenol, chloroform extraction techniques can direct the incorporation of radiolabeled amino acids into proteins using a cell-free protein-synthesizing system derived from wheat germ embryos, higher concentrations inhibit the translation of endogenous wheat germ mRNA, or added rabbit globin mRNA or polyribouridylic acid. This inhibitory activity is separated from poly(A)-rich RNA by oligo(dT)-cellulose chromatography. The inhibitory activity appears to reside in a heat-stable protein since it is inactivated by incubation with various proteases but not by DNAase I, pancreatic ribonuclease, alkaline hydrolysis, or treatment with formamide. Specificity of the inhibition is suggested since the inhibitory fraction prepared from GH3 cells also inhibits protein synthesis in a cell-free protein-synthesizing system derived from porcine parotid gland, while the inhibitory fraction prepared from porcine parotid gland has no inhibitory activity in this homologous system. Radioiodination and dodecylsulphate/polyacrylamide gel electrophoresis reveal several protein bands, the most prominent with an apparent molecular weight of 78 000.

Isolation and characterization of collagen messenger RNA*

Chick embryo collagen-synthesizing polysomes were isolated by differential centrifugation. RNA extracted from these particles was chromatographed in oligo(dT)-cellulose solumns and the mRNA thus obtained characterized as collagen mRNA by its electrophoetical mobility in acrylamide gels (equivalent to 1.05 x 10-6 daltons) and its effect upon a cell-free system derived from Krebs ascites tumor cells. The incorporation of 3H-proline was markedly dependent upon rabbit reticulocyte initiation factors and inhibited by initiation inhibitors such as aurintricaboxilate and pyrocatechol violet. The incorporation product was characterized as collagen by its lack of tryptophan, digestibility by purified bacterial collagenase, and by its co-chromatography with unlabled chick collagen in Sephadex G-200 and CM-cellulose columns.

Unique and repetitive sequences in multiple genes for feather keratin

Embryonic chick feather keratins are a family of homologous polypeptide chains. The mRNA coding for these has been obtained in a pure state and transcribed into complementary DNA (cDNA) using the reverse transcriptase from avian myeloblastosis virus. Studies on the kinetics of hybridisation and reannealing of cDNA indicate that there are 25-35 different keratin mRNA species in the embryonic chick feather, and a total of 100-240 keratin genes in the chick genome. Each keratin gene contains both a unique and a repetitive sequence. It is proposed that the repetitive sequences are the keratin coding sequences and that the unique sequences correspond to untranslated regions.

Messenger RNA metabolism of animal cells. Possible involvement of untranslated sequences and mRNA-associated proteins

The past several years have seen a virtual revolution in the study of eukaryotic mRNA. Among the notable recent achievements are the positive identification of mRNA precursors in HnRNA, the enumeration of the DNA sequences from which mRNA is transcribed, and the finding that mRNA in cultured cells is much more stable than was previously believed. One of most far-reaching discoveries has been the finding that mRNA in eukaryotes contains poly A. This discovery, aside from providing a powerful tool for mRNA isolation, has generated a large body of research into the properties and metabolism of poly A itself. In addition, the finding of a poly A-associated protein has given a renewed stimulus to the study of proteins associated with mRNA. This review is devoted to a discussion of these and related achievements, and some of their implications

Isolation and in vitro translation of delta-crystallin mRNA from embryonic chick lens fibers

Of the protein synthesized and accumulated during differentiation of embryonic chick lens fibers, 70-80% is the tissue specific protein delta-crystallin. We have isolated and partially characterized the total cytoplasmic mRNA from purified lens fibers of 15-day-old embryos as an initial step toward understanding the regulated expression of delta-crystallin during development. Each lens fiber mass contained an average of 10 mug of cytoplasmic RNA; approximately 0.1 mug per fiber mass was recovered in the mRNA fraction by oligo(dT)-cellulose chromatography. The mRNA electrophoresed primarily as a single peak on a polyacrylamide-agarose gel with an apparent molecular weight of about 9 x 10(5) estimated by comparison with 28S and 18S rRNA markers. Of the protein synthesized in response to the mRNA in cell-free systems derived from Krebs II ascites tumor cells or rabbit reticulocytes, 70-80% comigrated with delta-crystallin on sodium dodecyl sulfatepolyacrylamide-agarose gels. Comparison of the tryptic peptides of delta-crystallin with those of the in vitro products from both heterologous systems established that the lens fiber mRNA contained delta-crystallin mRNA, and that no other functional mRNAs were present in detectable quantities. Thus, the specialization of protein synthesis in embryonic chick lens fibers apparently results from an accumulation of delta-crystallin mRNA in the cytoplasm.