Effect of estrogen on gene expression in the chick oviduct. Effect of estrogen on the sequence and population complexity of chick oviduct poly(A)-containing RNA

Dynamic Expression Profile, Regulatory Mechanism and Correlation with Egg-laying Performance of ACSF Gene Family in Chicken (Gallus gallus)

Acyl-CoA synthetases (ACSs) are responsible for acyl-CoA synthesis from nonpolar hydrophilic fatty acids and play a vital role in many metabolic processes. As a category of ACS isozymes, members of ACS family (ACSF1-3) participate in lipid metabolism; however, their expression patterns, regulatory mechanisms and effects on egg-laying performance in chicken are poorly understood. Our in vivo and in vitro studies showed that ACSF1-3 genes were extensively expressed, and their expression levels changed dynamically in the liver among different development stages. Moreover, ACSF1 expression was upregulated and ACSF2 expression was downregulated by estrogen, but ACSF3 showed no response to estrogen treatment. The regulatory effect of estrogen on ACSF1 expression was mediated via ERα. The ACSF2 was highly expressed in the liver in peak-laying hens compared with pre-laying and late-laying hens, and also highly expressed in the liver continued egg-laying hens compared with inactive egg-laying hens. It is suggested that hepatic ACSF2 expression level might relate to egg-laying performance in chicken. In conclusion, the expression of ACSF1 was upregulated by estrogen via ERα, and the expression of ACSF2 was downregulated by estrogen and might be related to egg-laying performance in chicken.

Complexities and sequence similarities of mRNA populations of cholinergic (NS20-Y) and adrenergic (N1E-115) murine neuroblastoma cell lines

The clonal murine neuroblastoma cell lines NS20-Y and N1E-115 have been proposed as models for examining the commitment of neural crest cells to either the cholinergic or adrenergic phenotype, respectively. The validity of this model depends in part on the extent to which these two cell lines have diverged as a result of their transformed, rather than neuronal properties. In order to quantitate differences in gene expression between NS20-Y and N1E-115 cells, the mRNA complexity of each cell type was determined. An analysis of the kinetics of hybridization of NS20-Y cell mRNA with cDNA prepared from NS20-Y cell mRNA demonstrated the presence of approximately 11,700 mRNA species assuming an average length of 1900 nucleotides. A similar analysis using mRNA isolated from N1E-115 cells and cDNA prepared from N1E-115 cell mRNA demonstrated that the adrenergic cell line expressed approximately 11,600 mRNA species. The species of mRNA expressed by each cell line were resolved into high, intermediate, and low abundance populations. In order to determine whether mRNAs were expressed by the cholinergic, but not by the adrenergic cell line, NS20-Y cDNA was hybridized to an excess of N1E-115 cell mRNA. An analysis of the solution hybridization kinetics from this procedure demonstrated that the two cell lines do not differ significantly in the nucleotide complexity of their mRNA populations. The extensive similarity between the two mRNA populations suggests that only a small number of genes are expressed differentially between the two cell lines and supports their use as models for the differentiation of cholinergic and adrenergic neurons.

Messenger RNA content and complexity in normal and overloaded rat heart

Cardiac overload was studied in rats by abdominal aortic constriction, which increased the left ventricular weight by 59% after 12 days. During the transition, which precedes the compensatory hypertrophy, both total RNA and poly(A)-containing messenger RNA increased. The concentration of these polynucleotides peaked by day 4 after constriction, rising from 1.27 +/- 0.3 to 1.88 +/- 0.2 mg g-1 fresh weight for total RNA, and from 38 +/- 24 to 62 +/- 12 micrograms g-1 fresh weight for poly(A)-containing RNA, and returned to normal by day 12. However, the total amount per ventricle of both RNAs remained high. Poly(A)-containing RNA prepared from normal heart was hybridized to its cDNA copy. These results were expressed as the percentage of hybridization vs. the log 10 of the product of the poly(A)-containing RNA concentration and the time (Rot), and in computer analysis were described by division into three different frequency components. In normal hearts, the Rot 1/2 values of these components were, respectively, 3.98 X 10(-3), 0.338 and 21.380 mol.s.1(-1), which correspond to 2-3, 240 and 12,200 different sequences that were copied 22,000-33,000, 310 and 5 times, respectively. Four and 30 days after banding there was a harmonious enhancement of the number of the copies without any change in the number of different sequences, and the three different hybridization curves were superimposed. In conclusion, cardiac overload raises the poly(A)-containing RNA concentration, probably by stimulating transcription, but no major changes occur in any of the frequency classes.

Changes in plasma albumin concentration, synthesis rate, and mRNA level during acute inflammation

Induction of acute inflammation in rats by the subcutaneous injection of turpentine resulted in a marked fall in the concentration of albumin in plasma. This fall, which reached a minimum of 45% of the control level at 36 h after injection of the inflammatory agent, occurred in the presence of no significant change in the concentration of total protein in plasma. It was accompanied by a corresponding decline in the relative abundance of albumin mRNA in liver, which reached a minimum of 25% of the control level at 36 h after initiation of the inflammatory response. Perfused livers from 36-h postinjection rats exhibited albumin secretion rates that were reduced to 38% of control values. In contrast, release of total secretory proteins, secretion of nonalbumin plasma proteins, and synthesis of nonexported proteins by perfused livers were elevated to 166, 266, and 117% of the control values, respectively, as a result of the inflammatory response. These results demonstrate that acute inflammation causes a relative reduction in hepatic albumin mRNA, which leads to a corresponding decrease in albumin synthesis and secretion by liver and a fall in the concentration of albumin in plasma. The concentration of total protein in plasma is maintained during acute inflammation in part by increased synthesis and secretion by liver of nonalbumin plasma proteins, e.g. the acute-phase reactants.

Cloning of total mRNA populations from adult and embryonic mice

Total clone banks of cDNAs synthesized from poly(A)-RNA obtained from three stages of the developing mouse were constructed. The stages chosen were 13-day-old embryo, neonatal, and fully grown adult. To have as complete a bank as possible, large numbers of individual clones were generated approximately 400,000 for the 13th day embryo and neonatal mouse and approximately 610,000 for the adult bank. In each case the clone bank was constructed by inserting double stranded cDNA into the PstI site of pBR322 by the "G-C tailing" method. Sequences cloned in this way could be separated from the plasmid host DNA by treatment of the resultant total chimeric plasmid population with PstI. Aliquots of the cloned cDNA material were labeled with 32P by "nick translation" using Escherichia coli DNA polymerase I for the preparation of hybridization probes. Back-hybridization of these probes to the total clone banks allowed the determination of the sequence diversity among the above three very different developmental stages. The use of such clone banks should allow the identification of developmental stage specific mRNAs.

Responses to androgens of rat ventral prostate nuclear androgen-binding sites sensitive and resistant to micrococcal nuclease

Rat ventral prostate nuclei were separated into three major fractions by mild digestion with micrococcal nuclease and two fractions by extensive digestion. All fractions contained androgen-binding sites. Almost 50% of nuclear binding sites were resistant to enzymic digestion when only 5-15% of total DNA was resistant. Under milder digestion conditions, 21% of nuclear binding sites were associated with an intermediate fraction, representing 16% of total nuclear DNA, which was enriched in specific androgen-regulated gene sequences. This fraction was rapidly degraded by more extensive digestion. The nuclease sensitivity of these particular genes was markedly influenced by castration and the administration of dihydrotestosterone to castrated animals. The nuclear content of both nuclease-resistant and -sensitive androgen-binding sites was decreased by castration. Whereas the administration of androgen to animals castrated 1 day previously preferentially replenished nuclease-resistant sites, nuclease-sensitive sites, including those associated with transcriptionally active regions, had apparent priority when androgen was supplied to animals castrated 7 days previously. The significance of these observations to the regulation of nuclear processes and the possible interrelationships of nuclease-sensitive and -insensitive sites are discussed.

Segregation of specific classes of messenger RNA into free and membrane-bound polysomes

Analysis of mRNA populations from rat liver rough microsomes and free polysomes by homologous and heterologous cDNA . mRNA hybridization shows that the two mRNA populations are distinct, demonstrating that specific mRNA classes are efficiently segregated for translation in association with endoplasmic reticulum membranes. We estimate that approximately 90% of the mRNA in membrane-bound polysomes contains a diverse set of messengers with a minimum of 500--2000 different species although approximately 5--8 messengers may constitute 25--30% of the mRNA mass. The complexity of the mRNA population of free polysomes appears to be comparable to that estimated for total liver poly(A) + mRNA by other investigators, and is likely to be substantially greater than that of the bulk of bound mRNA. In addition, mRNA in free polysomes lacks the high abundance class characteristic of mRNA-bound polysomes. The substantial complexity of the bound mRNA population suggests that the segregation of polysomes in rough microsomes is not limited to a small class specialized in manufacturing secretory proteins, but extends to polysomes engaged in the synthesis of proteins for intracellular distribution. The segregation of specific messengers into the free and membrane-bound classes was abolished when polysome disassembly was induced by administration of ethionine. Thus, messenger RNA molecules themselves lacked the capacity for segregation, although they contain information for segregation which is expressed during translation. These findings are consistent with the presence of signal sequences in nascent polypeptides which determine the attachment of ribosomes to endoplasmic reticulum membranes.

Insulin gene expression during development of the fetal bovine pancreas

Poly(A+) RNA was isolated from the bovine pancreas at three stages of fetal development. Approximately 1% of the total RNA from first, second, and third trimester fetuses was polyadenylated, and the mean chain length of each RNA population was 1350 nucleotides. In cell-free protein synthesis experiments the concentration of insulin-immunoreactive translation products was 10.2%, 11.3%, and 9.7% for first, second, and third trimesters, respectively. Insulin mRNA sequences were estimated by transcription of insulin mRNA to [3H]cDNA and hybridization of cDNA with plasmid pI19 DNA containing rat proinsulin I sequences. Hybridization experiments gave insulin mRNA concentrations of 7.6%, 12.9%, and 3.9% for first, second, and third trimesters, respectively. These results show that insulin mRNA levels vary during development and become proportionally lower in third trimester, when the exocrine tissue is rapidly increasing in mass.

Problems associated with dose response in steroid-hormone activation of structural genes

A typical target cell for a sex steroid hormone contains 10 000--20 000 specific high-affinity receptors for that hormone. However full physiological responses can be achieved with only 2000 of these receptors involved in hormone--receptor complex interaction with the nucleus. The number of nuclear acceptor sites that must be filled before responses occur maybe even less. This implies that multiple occupation of nuclear acceptor sites by hormone--receptor may occur permitting co-operative induction of transcription of selected genes. The numbers of sites of initiation of RNA synthesis seem excessively high (about 70 000 per cell). Although this may be an artifact of the isolation procedures the proportion of initiation sites under hormonal control (equivalent to about 30 000 per cell) is still large. The numbers of mRNA species under hormonal control varies greatly depending on the particular hormone and target tissue. The extent to which these different observations can be incorporated into a unifying theory is discussed.

Purification of major abundance class of poly(A+)-RNA from rat ventral prostate

Poly(A+)--RNA from rat ventral prostate was isolated using oligo(dT)-cellulose chromatography. 45% of the total poly(A+)--RNA was a single peak at 10S as demonstrated by centrifugation in a 5-20% sucrose gradient containing 1% SDS. By using complementary DNA probes, it was shown that the 10S RNA contained the major abundance class of poly(A+)--RNA. Denaturing agarose-gel analysis revealed 2 major bands in the 10S poly(A+)--RNA preparation approx. 600 NT and 500 NT (NT = nucleotides) long, resp. Double-stranded 32 P-DNAs complementary to light side and heavy side of the 10S poly(A+)--RNA peak were synthesized and isolated using reverse transcriptase and hydroxyapatite (HAP) chromatography. Approx. 40% of the first strand of the cDNAs were converted to double-stranded structures with a Tm of 88 degrees C. HAP purified double-stranded material was 92% resistant to S1 nuclease. the DNA--DNA reannealing profile of double stranded 32 P-cDNA enriched for the 500 NT band gave a Cot 1/2 of approximately 7 X 10(-4) moles X sec X 1(-1) indicating a complexity for this enriched synthetic gene of 500-600 nucleotide pairs (NTP).

Androgenic regulation of messenger RNA sequence complexity in accessory sexual tissues of the male rat studied with fractionated complementary DNA

Effects of androgens on mRNA sequence complexity in the rat seminal vesicle have been investigated using complementary DNA fractionated on the basis of sequence abundance. Total cDNA complementary to poly(A)-rich RNA from normal rats was hybridised with an excess of the same RNA to controlled rot values and then the free cDNA was separated from cDNA . RNA hybrids by hydroxyapatite chromatography. Three cDNA fractions were obtained with very different hybridisation characteristics. Abundant cDNA hybridised to an excess of its parental RNA with an rot 1/2 of 2.46 x 10(-3) mol 1(-1) s and is complementary to about six or seven average-sized sequences. Use of hybrid-arrested translation in a cell-free protein-synthesising system has shown that this class of mRNA includes mRNAs coding for major androgen-dependent secretory proteins. Moderate and scarce cDNA fractions each showed more complex hybridization kinetics; computer analysis suggested each is complementary to two groups of average-sized sequences. Each cDNA fraction was hybridised to excess poly(A)-rich RNA from normal or castrated rats and the kinetics compared. Castration had no effect on the total number of sequences present in any class and did not alter the relative concentration of the scarce sequences. A small (threefold) decrease was seen in the concentration of abundant sequences with a larger (tenfold) decrease in the moderate class. Both de-reases were reversed by testosterone in vivo. The results are consistent with earlier studies where the effects of testosterone on seminal vesicle mRNA were followed using a translation assay and confirm that no gross differential effects are exerted on abundant mRNA coding for major secretory proteins. The cDNA fractions were also used to investigate the overlap in genetic expression between seminal vesicle and ventral prostate. Both tissues share all the scarce sequences in the same relative abundance. Less than 0.0015% and 0.004% of prostatic mRNA is complementary to seminal vesicle abundant and moderate sequences respectively. Similarly prostatic abundant sequences account for less than 0.004% of seminal vesicle mRNA.

Sequence complexity of bovine pituitary poly(A)RNA

Poly(A)RNA extracted from the anterior lobe of bovine-pituitary tissue was transcribed into its complementary DNA with reverse transcriptase. This 3H-labeled cDNA was hybridized with its template RNA. Hybridization kinetics revealed at least 3 abundance classes with the highest abundance class consisting of only a few different sequences. Bovine-liver poly(A)RNA did not contain this highest abundance class when hybridized to the cDNA probe complementary to pituitary poly(A)RNA. This result suggested that the highest abundance class found in bovine-pituitary poly(A)RNA was specific for that tissue and most likely contained the mRNA sequences for the major pituitary hormones.

Kinetic complexity of nuclear poly(A)-containing RNA in normal and regenerating rat liver

Poly(A)-containing RNA isolated from liver nuclei of untreated rats and 3 h or 12 h after partial hepatectomy or sham operation was hybridized to the complementary DNAs (cDNAs). In the homologous reactions two major components could be seen. When compared to normal liver, the complexity of the least abundant class was lower in nuclei from livers 3 h after partial hepatectomy and was higher in those isolated 12 h after operation. The heterologous reactions revealed an increase of some abundant poly(A)-containing sequences and a loss or dilution of rare sequences 3 h after operation. The latter effect was not specific to the regeneration process but occurred after laparotomy as well. 12 h after partial hepatectomy, however, about 10% new poly(A)-containing sequences were detected, corresponding to about 5000 molecules of 4500 nucleotides length, which are unique to regenerating nuclei.

Analysis of the genetic complexity and abundance classes of messenger RNA in human liver and leukemic cells

In an effort to determine the number of genes expressed as messenger RNA in disparate human tissues we have analyzed the genetic complexity of the polyribosome-associated poly(A)-containing RNA population obtained from liver and lymphoblastic leukemic cells. This was accomplished by measuring the kinetics of hybridization of mRNA to a complementary DNA probe synthesized by avian myeloblastosis virus reverse transcriptase in vitro. The results obtained from such an analysis revealed the presence of two major abundance classes of mRNA with a total genetic complexity of approximately 10,000 diverse mRNA species in both of these cell types. Diversity of mRNA species in these unrelated human cells was studied by heterologous hybridization reactions between the cDNA of one cell type and a vast excess of poly(A)-containing mRNA from another. These types of studies indicated that extensive homology (more than 80%) exists in the mRNA sequences of disparate human cell types and suggest that the vast majority of genetic information expressed as mRNA is required for the maintenance of cellular functions common to functionally different human tissues.

The chick ovomucoid gene contains at least six intervening sequences

A 15-kilobase pair EcoRI chick DNA fragment, containing both the termination codon UGA and the 5'-portion of the structural ovomucoid gene, has been cloned in lambda phage Charon 4A by in vitro packaging. Restriction mapping and electron microscopic analyses of this cloned DNA have revealed that the structural ovomucoid gene sequences are separated by at least six intervening sequences.

Transcription of structural and intervening sequences in the ovalbumin gene and identification of potential ovalbumin mRNA precursors

Structural sequences that are extensively separated by nonstructural intervening sequences in the natural ovalbumin gene are coordinately expressed in target and nontarget tissue. The intervening sequences, which consist of unique sequences in the chick genome, are transcribed in their entirety. The amount of nuclear RNA corresponding to these sequences, however, is approximately 10 times less than that observed for structural sequences. The accumulation of RNA corresponding to structural and intervening sequences during acute estrogen stimulation suggests either that there are different rates of transcription for these regions of the ovalbumin gene or that RNA sequences corresponding to the intervening sequences are preferentially processed and degraded. Comparison of the in vitro expression of portions of the ovalbumin gene in nuclei isolated from chronically stimulated oviducts indicates that both structural and intervening sequences are preferentially transcribed in vitro at rates approximately 500 times greater than expected for random transcription of the haploid chick genome. In addition, electrophoresis of oviduct nuclear RNA on agarose gels containing methylmercury hydroxide reveals multiple species of RNA that are from 1.3 to over 4 times larger than ovalbumin mRNA and hybridize to both structural and intervening sequences of the ovalbumin gene. These results are consistent with transcription of the entire ovalbumin gene into a large precursor molecule followed by excision of the intervening sequences and appropriate ligation of the structural sequences to form the mature mRNA.

Studies on the binding of the estradiol-receptor complex to rat DNA fragments

The activation by estradiol of a series of reactions in hormone target tissues provides a model for the study of gene regulation in eukaryotic cells. We have examined the interaction of the estradiol--receptor complex with the various frequency classes of rat DNA to determine whether specificity can be detected at this level of chromosome organization. Interaction of the 8S estradiol--receptor complex with DNA was assayed by a change in the sedimentation rate of the complex. The receptor was found to bind to high molecular weight rat DNA as well as to 450 nucleotide long fragments of rat DNA. When the sheared DNA was separated by denaturation and reassociation into three frequency classes, the unique and moderately repeated sequences bound the receptor almost as effectively as the total sheared DNA. The resulting DNA--receptor complexes were sensitive to pancreatic DNAase. The highly repeated class of sequences bound the receptor less effectively than the other fractions. This did not result from the presence in this fraction of an inhibitor of binding, since addition of unique DNA fragments resulted in formation of a normal DNA--receptor complex. Nor was nuclease activity responsible since intact DNA could be isolated from the incubation mixture. An increase in length of the highly repeated DNA to 800 nucleotide long fragments caused them to bind the receptor almost as effectively as the other DNA fractions. These studies suggest that there may be sequences in eukaryotic DNA that preferentially bind the estradiol--receptor complex.

Isolation and characterization of poly(adenylic acid)-containing messenger ribonucleic acid from rat liver polysomes

Undegraded rat liver polysomes were obtained after homogenizing the tissue in a medium containing NH4Cl, heparine, and yeast tRNA. Purification of poly(A)-containing RNA from polysomal RNA was accomplished by affinity chromatography on oligo(dT)-cellulose columns. Poly(A)-containing RNA molecules were monitored by the formation of ribonuclease-resistant hybrids with [3H]poly(U). To improve the separation of messenger RNA and ribosomal RNA by oligo(dT)-cellulose it was found essential to dissociate the aggregates formed between both molecular species by heat treatment in the presence of dimethylsulfoxide (Me2SO) prior to chromatography. Sucrose gradient analysis under denaturing conditions showed that the preparations obtained were virtually free of ribosomal RNA. Poly(A)-containing RNA constituted approx. 2.2% of the total polysomal RNA and the number average size was 1500--1800 nucleotides, as judged by sedimentation analysis on sucrose density gradients containing Me2SO. Approximately 8.2% of the purified preparation obtained was able to anneal with [3H]poly(U); the number average nucleotide length of the poly(A) segment of the RNA population was calculated to be 133 adenylate residues. Based on these values, our preparations appear to be greater than 90% pure. The RNA fractions obtained after oligo(dT)-cellulose chromatography were used to direct the synthesis of liver polypeptides in a heterologous cell-free system derived from wheat-germ. The system was optimized with respect to monovalent and divalent cations, and presence of polyamines (spermine). More than 65% of the translational activity present in the unfractionated polysomal RNA was recovered in the final poly(A)-containing RNA fraction. However, about 25% of the activity was found to be associated with the unbound fraction which was essentially free of poly(A)-containing RNA. Immunoprecipitation analysis with a specific antiserum to rat serum albumin demonstrated that about 6--8% of the labeled synthetic products translated from the poly(A)-containing RNA sample corresponded to serum albumin. Analysis of the translation products by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a heterogeneous distribution of molecular sizes ranging from 15 000 to greater than 70 000 daltons. Spermine not only increased the overall yield and extent of protein synthesis, but also resulted in higher yields of large protein products. Under optimal translation conditions a discrete peak representing about 7% of the total radioactivity was observed to migrate with rat serum albumin.

Hormonal control of mRNA synthesis studied by nucleic acid hybridization

The regulation of protein synthesis by steroids is thought to be due to hormonal effects primarily on mRNA concentration. Experimental evidence to support this conclusion has come largely from the use of DNA probes complementary (cDNA) to mRNA molecules or by translation of the mRNA in vitro. In this review the experimental procedures involved and the application to hormone action of cDNA hybridization will be reviewed. (1) mRNA concentrations can be assayed in tissue RNA samples by hybridization with radiolabelled complementary DNA probes (cDNA). From the rate of hybridization of an mRNA preparation to a cDNA probe it is possible to estimate specific mRNA concentrations and thereby study their hormonal regulation within tissues of subcellular fractions (2) Rates of synthesis of a specific RNA can be measured by hybridization of pulse-labelled RNA with excess cold cDNA as illustrated in studies of the glucocorticoid induction of MMTV RNA. (3) Hormore-induced alterations of mRNA populations as a whole can be investigated. From the kinetics of hybridization of mRNA with its complementary DNA it is possible to estimate the number of different RNA sequences in tissues and to approximate the number of copies of each sequence per cell. Consequently, by comparing mRNA samples isolated from tissues of different hormonal status it is possible to demonstrate specific hormone-inducible mRNA species and, in some cases, identify their translation products.

Isolation and characterization of purified rat casein messenger ribonucleic acids

Purification of casein messenger ribonucleic acids (mRNAs) from lactating rat mammary gland RNA has been accomplished by a combination of sizing techniques, including Sepharose 4B chromatography and preparative agarose-urea gel electrophoresis, and affinity chromatography of poly(adenylic acid)-containing mRNA on oligo(dT)-cellulose. The separation of the individual casein mRNAs into discrete molecular species free of apparent ribosomal RNA contaminants was facilitated by the use of denaturing conditions either prior to or during each of the fractionation procedures. Two casein mRNA fractions were isolated: (1) a 15S mRNA doublet which directed the synthesis of the two largest rat caseins in the wheat-germ, cell-free, translation assay, and (2) a 12S mRNA which migrated as a single species during agarose-urea gel electrophoresis and directed the in vitro synthesis of the smallest of three rat caseins. These mRNAs had apparent molecular weights of 450 000 +/- 30 000 and 320 000 +/- 25 000 and contained poly(adenylic acid) sequences at their 3' termini ranging from 15 to 150 residues with number average lengths of 42 and 38 adenosines, respectively. The purity of the isolated casein mRNA'S was determined both by agarose-urea gel electrophoresis and by a careful comparison of the total products synthesized in the wheat-germ translation assay with those recognized by a specific casein antibody using an indirect immunoprecipitation technique. The specificity of the indirect immunoassay procedure was demonstrated by the selective displacement by purified rat casein of greater than 95% of the radioactive product synthesized in the cell-free system. Under optimal translation conditions for casein mRNA, at least 90% of the released protein synthesized in response to the 15S casein mRNA was specifically immunoprecipitable, representing a 178-fold purification compared with the initial RNA extract. Using these techniques a comparable purification was also obtained for a 15S mouse casein mRNA fraction. Finally, an analysis by fluorography on 5-20% (w/v) polyacrylamide gradient slab gels of the total proteins synthesized in response to both the 15S and 12S casein mRNAs revealed a close correspondence with those proteins which were specifically immunoprecipitated.