Unique and repetitive sequences in multiple genes for feather keratin

Development-Associated Genes of the Epidermal Differentiation Complex (EDC)

The epidermal differentiation complex (EDC) is a cluster of genes that encode protein components of the outermost layers of the epidermis in mammals, reptiles and birds. The development of the stratified epidermis from a single-layered ectoderm involves an embryo-specific superficial cell layer, the periderm. An additional layer, the subperiderm, develops in crocodilians and over scutate scales of birds. Here, we review the expression of EDC genes during embryonic development. Several EDC genes are expressed predominantly or exclusively in embryo-specific cell layers, whereas others are confined to the epidermal layers that are maintained in postnatal skin. The S100 fused-type proteins scaffoldin and trichohyalin are expressed in the avian and mammalian periderm, respectively. Scaffoldin forms the so-called periderm granules, which are histological markers of the periderm in birds. Epidermal differentiation cysteine-rich protein (EDCRP) and epidermal differentiation protein containing DPCC motifs (EDDM) are expressed in the avian subperiderm where they are supposed to undergo cross-linking via disulfide bonds. Furthermore, a histidine-rich epidermal differentiation protein and feather-type corneous beta-proteins, also known as beta-keratins, are expressed in the subperiderm. The accumulating evidence for roles of EDC genes in the development of the epidermis has implications on the evolutionary diversification of the skin in amniotes.

Dynamic evolution of the alpha (α) and beta (β) keratins has accompanied integument diversification and the adaptation of birds into novel lifestyles

BACKGROUND

Vertebrate skin appendages are constructed of keratins produced by multigene families. Alpha (α) keratins are found in all vertebrates, while beta (β) keratins are found exclusively in reptiles and birds. We have studied the molecular evolution of these gene families in the genomes of 48 phylogenetically diverse birds and their expression in the scales and feathers of the chicken.

RESULTS

We found that the total number of α-keratins is lower in birds than mammals and non-avian reptiles, yet two α-keratin genes (KRT42 and KRT75) have expanded in birds. The β-keratins, however, demonstrate a dynamic evolution associated with avian lifestyle. The avian specific feather β-keratins comprise a large majority of the total number of β-keratins, but independently derived lineages of aquatic and predatory birds have smaller proportions of feather β-keratin genes and larger proportions of keratinocyte β-keratin genes. Additionally, birds of prey have a larger proportion of claw β-keratins. Analysis of α- and β-keratin expression during development of chicken scales and feathers demonstrates that while α-keratins are expressed in these tissues, the number and magnitude of expressed β-keratin genes far exceeds that of α-keratins.

CONCLUSIONS

These results support the view that the number of α- and β-keratin genes expressed, the proportion of the β-keratin subfamily genes expressed and the diversification of the β-keratin genes have been important for the evolution of the feather and the adaptation of birds into multiple ecological niches.

Comparative studies on two satellite RNAs of cucumber mosaic virus

Cucumber mosaic virus (CMV) satellite RNA (Sat-RNA, D. W. Mossop and R. I. B. Francki, 1978, Virology86, 562-566) is similar in many of its physical and biological properties to CMV associated RNA 5 (CARNA 5) described by Kaper and Tousignant (1977, Virology85, 323-327). However, CARNA 5, unlike Sat-RNA, causes a serious necrotic disease of tomato. Sat-RNA when inoculated together with various CMV or tomato aspermy virus strains not only failed to increase the severity of symptoms in infected tomato plants, but ameliorated them in some instances. Comparisons of the two RNAs by hybridization analysis using 32P-labelled complementary DNA probes, indicate that they have partial nucleotide sequence homology. It is suggested that the difference in their primary structure is reflected in their biological properties.

Avian scale development

Germinative cells of the scutate scale epidermis from 15-day embryos are committed to appendage-specific, beta stratum formation in association with a foreign dermis. Commitment precedes the time (17 days of development) at which beta strata are actually present in their site-specific locations along the outer surface of each scutate scale. This observation suggested the possibility that commitment to beta stratum formation might be occurring as the outer epidermal surface of each scutate scale first becomes established between 12 and 13 days of development. It is at this time that the scale epidermis loses its ability to participate in feather morphogenesis and cell proliferation becomes restricted to a true stratum basale. To examined the ability of the presumptive scutate scale epidermis to generate beta strata in the absence of the inductive scutate scale dermis, scutate scale epidermis from 11-, 12-, and 13-day embryos was recombined with 15-day reticulate scale dermis and grown for 7 or 9 days. The dermis of reticulate scales does not induce beta stratum formation, but it does support differentiation of a beta stratum by the determined 15-day scutate scale epidermis. Using immunohistological and biochemical analyses of beta-keratins, we find that each of these presumptive scutate scale epidermises is competent to generate appendage-specific beta strata in the absence of the scutate scale dermis. This determination is occurring prior to scale ridge morphogenesis and differentiation of the epidermis into the distinct outer and inner epidermal surfaces of the scale ridge. The restricted distribution of beta strata to the apical domes of individual reticulate-like scales demonstrates that the germinative cells of the committed epidermises are responding to patterned cues. This study also suggests that all basal cells of the presumptive scutate scale epidermis are initially endowed with the ability to generate cells that form a beta stratum.

Avian keratin genes. II. Chromosomal arrangement and close linkage of three gene families

We describe the isolation and characterization of a set of overlapping cosmid clones that contain chicken keratin genes. The 100 kb (1 kb = 10(3) base-pairs) of DNA represented in these clones contains a cluster of 18 feather keratin genes spanning 53 kb of DNA. The feather keratin genes are spaced about 3 kb apart and at least 11 of them have the same transcriptional orientation. Southern analysis using oligonucleotide probes made from highly conserved portions of the 5' non-coding, intron and 3' non-coding regions, respectively, indicate that these sequences have been highly conserved among the gene family as a whole, with only one or two exceptions in each case. The presence of some regularly repeated restriction enzyme sites are indicative of tandem duplication events in the recent history of the feather keratin gene family. The feather keratin gene locus is flanked on both sides by related types of keratin genes. On the 5' side of the feather gene cluster are three keratin genes (designated feather-like) that are located 5 kb from the last feather keratin gene and are spaced about 4 kb apart. On the 3' side of the feather gene cluster, 21 kb from the last feather keratin gene, lies a cluster of four genes that encode claw keratins. These genes are spaced about 1 kb apart and appear to be divergently transcribed. Partial DNA sequence analysis of the feather-like gene lying proximal to the feather keratin gene cluster demonstrated that it encodes a protein of 115 amino acid residues that is 80% homologous to the feather keratins at both the DNA and amino acid sequence levels. The feather-like gene(s) are expressed in both embryonic and adult (post-hatch) chick feathers and at a very low level in embryonic scale tissue. These genes therefore form a new family of feather proteins that is distinct from the previously characterized feather keratins.

Changes in histones H2A and H3 variant composition in differentiating and mature rat brain cortical neurons

Rat brain cortical neurons originate from germinal cells during a period of 6 days immediately before birth. Upon leaving the proliferative layer neurons become irreversibly quiescent. We have previously reported the presence of core histone nonallelic variants in terminally differentiated rat brain cortical neurons. Although the functional significance of core histone variants is unknown, several lines of evidence suggest that the processes of variant replacement could be involved in the structural and functional differentiation of chromatin. Here we describe the changes in core histone composition that occur during postnatal development. The changes in chromatin composition are already apparent at birth, suggesting that the change in synthesis patterns is related to the arrest of cell proliferation and neuron commitment. During postnatal development H2A.2, H2A.x, and H3.3 accumulate, whereas H2A.1, H3.1, and H3.2 decrease. H2A.z is the only variant that remains constant. The time courses of replacement and the observed variant proportions when the variant composition approaches the equilibrium suggest that all H2A variants are synthesized either in germinal cells or in neurons, whereas H3.1 and H3.2 seem to be synthesized only in germinal cells. The extent of the replacement of H3.1 and H3.2 by H3.3 shows that the exchange process affects most of the chromatin. The half-life times of H2A.1 and H3.2 were calculated from their respective exponential decays. Values of 65 days or less and 142 days were found for H2A.1 and H3.2, respectively. The preferential replacement of H2A.1 over H3.2 reinforces the view that the histone core does not degrade as a single unit.

Differentiation of embryonic chick feather-forming and scale-forming tissues in transfilter cultures

The dermal-epidermal tissue interaction in the chick embryo, leading to the formation of feathers and scales, provides a good experimental system to study the transfer between tissues of signals which specify cell type. At certain times in development, the dermis controls whether the epidermis forms feathers or scales, each of which are characterized by the synthesis of specific beta-keratins. In our culture system, a dermal effect on epidermal differentiation can still be observed, even when the tissues are separated by a Nuclepore filter, although development is abnormal. Epidermal morphological and histological differentiation in transfilter cultures are distinct and recognizable, more closely resembling feather or scale development, depending on the regional origin of the dermis. Differentiation is more advanced when epidermis is cultured transfilter from scale dermis than from feather dermis, as assessed by morphology and histology, as well as the expression of the tissue-specific gene products, the beta-keratins. Two-dimensional polyacrylamide gel analysis of the beta-keratins reveals that scale dermis cultured transfilter from either presumptive scale or feather epidermis induces the production of 7 of the 9 scale-specific beta-keratins that we have identified. Feather dermis, although less effective in activating the feather gene program when cultured transfilter from either presumptive feather or scale epidermis, is able to turn on the synthesis of 3 to 6 of the 18 feather-specific beta-keratins that we have identified. However, scale epidermis in transfilter recombinants with feather dermis also continues to synthesize many of the scale-specific beta-keratins. Using transmission and scanning electron microscopy, we detect no cell contact between tissues separated by a 0.2-micron pore diameter Nuclepore filter, while 0.4-micron filters readily permit cell processes to traverse the filter. We find that epidermal differentiation is the same with either pore size filter. Furthermore, we do not detect a basement membrane in transfilter cultures, implying that neither direct cell contact between dermis and epidermis, nor a basement membrane between the tissues is required for the extent of epidermal differentiation that we observe.

Isolation and characterisation of keratin mRNA from the scale epidermis of the embryonic chick

The keratin polypeptides of the epidermis from the leg scale region of 17-day-old embryonic chicks were extracted as S-carboxymethylated derivatives and characterised by electrophoresis on SDS and pH 9.5 urea gels including a combination of both in two dimensions. Proteins were isolated that gave X-ray diffraction patterns typical of alpha- and beta- (avian feather) keratins. An mRNA fraction was isolated from 17-day-old scale tissue by guanidinium chloride extraction and sucrose gradient fractionation. The mRNA was translated in the wheat germ system to give a major product indistinguishable from the molecular weight class (Mr 14 500) of scale beta-keratin polypeptides. A cDNA library was constructed in pBR322 from a 15 S mRNA subfraction and two recombinant clones were selected by their strong hybridisation to cDNA prepared from the 15 S mRNA. The sequencing of these has yielded details of the relatedness of two scale keratin genes including their 3' untranslated regions. Almost half of the protein sequences of the two homologous scale keratins has been deduced and a notable feature of the scale keratin structure appears to be the presence of at least two sequence domains consisting of 13 amino acid repeats.

Development of chicken epidermis cultured with embryo extract

The epidermis from 11-day-old chick embryo shank skin was cultured with 11-day-old chick embryo extract. The growth and the differentiation of the epidermis in culture were studied histologically, electron microscopically and with polyacrylamide gel electrophoresis of keratin proteins. The epidermis cultured with the chick embryo extract proliferated and stratum structures developed simultaneously with the increase in epidermal cell layers. Finally, a keratinized layer was observed after 10 days in culture. Electron microscopic observations revealed that tonofilaments were produced after 2 days in culture and increased thereafter with culture time, becoming condensed with desmosomes. Keratohyaline granules were observed in 7-day cultures. These keratinization characteristics occurring during culture showed the general characteristics of the alpha stratum observed in the skin of in ovo embryos during the early stages of development. However, the development of peridermal and subperidermal granules was poor and the stratum granulosum, which develops at the late stages between the stratum intermedium and the stratum corneum, was not observed. Polyacrylamide gel electrophoresis of S-carboxymethylated keratin proteins showed that the keratin protein band patterns of the culture differed from those of in ovo skin epidermis.

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.

Monoclonal antibodies provide specific intramolecular markers for the study of epithelial tonofilament organization

The tonofilament-associated protein antigens recognized in epithelial cells by a group of six monoclonal antibodies have been studied by immunofluorescence and gel immunoautoradiography. The monoclonal antibodies were generated against detergent insoluble cytoskeleton extracts from a cultured simple epithelium derived cell line, Ptk1 cells. They show various tissue specificities, and while they all recognize components at the low end of the molecular weight range for intermediate filament proteins, they confirm that single antibody species can react with multiple polypeptides of different molecular weights in the tonofilament complex. The monoclonal antibodies described here demonstrate the presence of a simple epithelium antigenic determinant associated with intermediate filaments that is not detectable in the specialized cells of squamous and keratinizing epithelia but can reappear in such cells after transformation.

Reiteration frequency of procollagen genes in the guinea pig genome. Collagen genes are not amplified during granuloma fibroblasts differentiation

Procollagen mRNA was purified from collagen synthesizing polysomes obtained from an experimental guinea pig granuloma, and iodinated in vitro. The procollagen 125I-labelled mRNA was hibridized with granuloma and liver guinea pig DNA in vast DNA excess conditions. A Cot 1/2 800-900 mol . s . 1-1 for both tissues was obtained from the hybridization curves. With these results, we could suggest the existence of 11-13 procollagen genes per haploid genome. By the analysis of the hybridization data it was possible to infer that there is no genomic amplification in tissues highly specialized in the synthesis of collagen such as granuloma.

Molecular organization of avian epidermal structures

1. A number of differences in the phi-keratins have been shown to exist among various avian epidermal structures. 2. There is extensive electrophoretic heterogeneity within tissues and two elemental sizes of monomers. Feathers, natal and definitive down monomers have a weight of 10,500 dalton; scale, break and claw monomers approx 13,500 daltons. 3. Differences in the size of the monomers is due to the presence of an insoluble tryptic peptide high in Gly, Phe, Leu and Tyr. 4. Tissue differences in composition are related to structure and function.

Mammalian epidermal messenger RNA: identification and characterization of the keratin messengers

A messenger RNA fraction which directs the synthesis of epidermal keratins and other skin proteins has been isolated from adult guinea pigs and newborn rats, utilizing techniques designed to minimize degradation by endogenous nucleases. During the initial extraction procedures an inhibitor of translation was identified. This inhibitor could be removed by sedimentation of the RNA through cesium chloride. Electrophoresis of the resulting RNA on denaturing agarose/urea gels indicated that, in addition to 18S and 28S ribosomal RNAs, several minor species ranging in size from 10S to 28S were present. This heterogeneous RNA stimulated the incorporation of radioactive amino acids into protein in a reticulocyte lysate system which had an absolute dependence on added mRNA. A fraction of the RNA was retained on oligo(dT)-cellulose, indicating the presence of poly(adenylic acid) sequences. This poly(A)-rich material was considerably enriched for mRNA activity. Analysis of the products of synthesis indicated that proteins which migrated as keratins in 1- and 2-dimensional electrophoretic systems were major translation products of both the unfractionated material and poly(A)-containing fractions. The minimum sedimentation coefficient of keratin mRNAs was found to be 18S, a value consistent with the molecular weight of the keratin polypeptides.

Intermediate filaments as mechanical integrators of cellular space

Five chemically distinct classes of intermediate filaments can be identified within higher eukaryotic cells. Each class is characteristic of a particular cell type. These filaments may function to integrate mechanically the various structures of the cytoplasmic space in a way that is tailored to the differentiated state of the cell.

Histone genes are clustered with a 15-kilobase repeat in the chicken genome

Histone mRNA isolated from 5-day-old chick embryos has been used as a template for complementary DNA (cDNA) synthesis. The resultant cDNA, after removal of sequences complementary to rRNA, was used to detect histone genes in adult chicken genomal DNA. Hybridisation data indicate that the histone genes are repeated about 10-fold in the chicken genome. Restriction endonuclease analysis reveals some sequence heterogeneity in these genes. However, the results show that chicken histone genes are clustered with a basic repeat unit of 15 kilobases.

Alfalfa mosaic virus RNA. Determination of the sequence homology between the four RNA species and a comparison with the four RNA species of cucumber mosaic virus

The method of Taylor et al. [Taylor, J. M., Illmensee, R & Summers, J. (1976) Biochim. Biophys. Acta, 442, 324--330 and Gould and Symons (1977) Nucleic Acids Res. 4, 3787--3802] has been used to transcribe complementary DNA probes from the four major RNAs of alfalfa mosaic virus (AMV). Analysis of the kinetics of hybridization of these probes in homologous and heterologous complementary DNA . RNA hybridization reactions has shown that the sequence of the smallest RNA (RNA 4), which contains the coat protein gene, is present within RNA 3 and located at the 3' end of this RNA species. RNAs 1 and 2 are unique RNA molecules with little or no sequence homology between them or RNAs 3 and 4. This latter observation contrasts with the situation that occurs in cucumber mosaic virus (CMV) as CMV RNAs 1--4 were shown to have a common nucleotide stretch of 200 bases at their 3' termini; the location of RNA 4 within RNA 3 of CMV was also shown to be at the 3' end of this RNA species.

Messenger RNA for the insect storage protein calliphorin: in vitro translation and chromosomal hybridization analyses of a 20 S poly(A)-RNA fraction

A major poly(A)-containing RNA fraction of the approximate size expected of a monocistronic mRNA for the storage protein calliphorin has been isolated from the larval fat bodies of Calliphora vicina during early instar 3. This 20 S RNA fraction programs the synthesis by cell-free wheat embryo extracts of polypeptides of 86,000 daltons identified by tryptic peptide fingerprinting as precursors of the authentic calliphorin subunits of 83,000 daltons. Complementary DNA synthesized by AMV reverse transcriptase using the same 20 S RNA as template hybridized in situ to a single segment of one or two bands in the salivary polytene chromosomes of C. vicina.

Globin messenger RNA content in hepatomas: a test of retrogenesis

The globin mRNA content of foetal, neonatal and normal adult liver and of the transplantable Morris hepatoma 5123C and host liver of animals carrying the tumour was measured using molecular hybridization of total nucleic acid extracts from these tissues with a complementary DNA copy of globin mRNA from rabbit reticulocytes. The purpose of these experiments was to determine, as a test at the molecular level of the retrogenesis hypothesis, if the gene for foetal globin is activated in neoplastic hepatic tissue or induced in host liver. These experiments did not detect any foetal globin mRNA in the hepatoma or host liver nucleic acid extracts in spite of the increase in haemopoietic activity induced in host animals as a consequence of tumour bearing.

Determination of the sequence homology between the four RNA species of cucumber mosaic virus by hybridization analysis with complementary DNA

The method of Taylor etal., (11) has been used to transcribe complementary DNA probes from the four major RNA species of cucumber mosaic virus (RNAs 1 - 4 in order of decreasing molecular weight). Analysis of the kinetics of hybridization of these probes in homologous and heterologous complementary DNA-RNA hybridization reactions has shown that the sequence of the smallest RNA (RNA 4), which contains the coat protein gene, is present within RNA 3. RNAs 1 and 2 are unique RNA molecules while each has a region of approximately 300 nucleotides in common with RNA 4.Images

The organization of repeated nucleotide sequences in the replicons of mammalian DNA

Chinese hamster ovary cells were irradiated with 100-5,000 rads of X-rays and inhibition of the initiation of replicons after irradiation was demonstrated by analyzing nascent DNA sedimented in alkaline sucrose gradients. The renaturation kinetics of DNA synthesized during 60 min of incubation after irradiation was compared with that of DNA synthesized during the 60 min after sham irradiation and with that of parental DNA. Nascent DNA from cells whose replicon initiation was inhibited renatured faster than nascent DNA from control cells in the COt range of repeated nucleotide sequences, suggesting that regions of the replicon not close to origins are enriched in repeated sequences and that regions close to origins are enriched in unique sequences. A class of repeated nucleotide sequences may be involved in the regulation of replicon initiation.

Organization of sequences of avian globin mRNA

Formamide polyacrylamide gel electrophoresis shows that chicken globin mRNA contains about 6.50 nucleotides, and since only 435 of these code for globin, a further 215 are not translated, and their function and position are not known. This work has produced the following conclusions. 1. 45-50 of these untranslated nucleotides are present as poly (A) at the 3' terminus. 2. The 3' untranslated region of chicken globin mRNA is at least 90 nucleotides in length. This minimal estimate is based on data derived from hybridization of defined lenghts of chicken globin cDNA to rabbit globin mRNA. The percentage of avian globin cDNA sequences which hybridize to rabbit globin mRNA is directly proportional to the length of the cDNA in each case. This relationship holds for lengths of cDNA from 115 up to 620 nucleotides. The low percentage homology for short cDNA molecules is not due to their being short per se. In homologous mRNA excess hybridizations (chicken cDNA/chicken mRNA), all cDNA preparations were completely protected from S1 nuclease digestion. 3. It is probable that there is greater evolutionary divergence in the 3' untranslated region of chicken and rabbit globin mRNA when compared with the coding regions of these molecules; The combined data is sued to formulate a regional map of chicken globin mRNA,

The keratin chains of avian scale tissue. Sequence heterogeneity and the number of scale keratin genes

The three major proteins of chick scale keratin were isolated as their S-carboxymethylated derivatives and shown to be similar or identical in molecular weight by gel filtration but to be distinct by amino acid analysis and gel electrophoresis. The major amino-terminal sequence of scale keratin chains was determined and shown to be highly homologous to the corresponding region of feather keratin chains. The carboxyl-terminal peptides of the three scale keratin fractions differed in sequence but were all homologous to the carboxyl-terminal segment of feather keratin chains. The pronounced concentration of cysteine residues at the amino-terminal and carboxyl-terminal segments suggested a similar role for these regions in both scale and feather keratin chains, namely to provide a disulphide-linked matrix to maintain the organisation of fibrils which arise from the internal hydrophobic segments of both types of chain. Analysis of a large hydrophobic segment from each of the three isolated protein fractions revealed that each was composed largely of repeating tripeptide units of the type Gly-Gly-X (where X = Phe, Leu or Tyr). At a few positions in each hydrophobic peptide, microheterogeneity was apparent in the sequences indicating that each isolated protein fraction was composed of at least three different chains each encoded by a different gene. A minimum of nine keratin genes are therefore expressed in scale tissue.

Distribution of globin genes in chicken reticulocyte chromatin fractionated on urografin gradients

Buoyant density centrifugation in Urografin solutions resolved French Pressure Cell-sheared, and micrococcal nuclease-digested avian reticulocyte chromatin into a broad profile of two peaks. Hybridization experiments using a globin cDNA probe suggested minimal fractionation of transcriptionally active and inactive components with chromatin sheared at 6000 psi, while no evidence was obtained for any fractionation with chromatin sheared at lower or higher pressures, or with chromatin digested to various extents with micrococcal nuclease, despite a considerable spread of chromatin material across gradients.

[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.

Reiteration frequency of the gene for tissue-specific histone H5 in the chicken genome

Chicken erythroid cells contain a tissue specific histone known as H5 in addition to the five major histone species found in other organisms. The mRNA coding for this histone has been isolated by indirect immunoprecipitation from immature, non-dividing reticulocytes in which this is the only histone synthesised. The mRNA has been modified by the enzymatic addition of a 3' polyadenylic acid tract, and transcribed into complementary DNA (cDNA) using the RNA-dependent DNA-polymerase from avian myeloblastosis virus. Studies on the hybridisation of this cDNA indicate that the gene coding for the H5 histone is reiterated 10 times in the chicken genome.