Isolation and characterization of the Bos taurus beta-casein gene

Cloning and Molecular Characterization of Porcine β-casein Gene (CNS2)

The production of therapeutic proteins from transgenic animals is one of the most important successes of animal biotechnology. Milk is presently the most mature system for production of therapeutic proteins from a transgenic animal. Specifically, β-casein is a major component of cow, goat and sheep milk, and its promoter has been used to regulate the expression of transgenic genes in the mammary gland of transgenic animals. Here, we cloned the porcine β-casein gene and analyzed the transcriptional activity of the promoter and intron 1 region of the porcine β-casein gene. Sequence inspection of the 5′-flanking region revealed potential DNA elements including SRY, CdxA, AML-a, GATA-3, GATA-1 and C/EBP β. In addition, the first intron of the porcine β-casein gene contained the transcriptional enhancers Oct-1, SRY, YY1, C/EBP β, and AP-1, as well as the retroviral TATA box. We estimated the transcriptional activity for the 5′-proximal region with or without intron 1 of the porcine β-casein gene in HC11 cells stimulated with lactogenic hormones. High transcriptional activity was obtained for the 5′-proximal region with intron 1 of the porcine β-casein gene. The β-casein gene containing the mutant TATA box (CATAAAA) was also cloned from another individual pig. Promoter activity of the luciferase vector containing the mutant TATA box was weaker than the same vector containing the normal TATA box. Taken together, these findings suggest that the transcription of porcine β-casein gene is regulated by lactogenic hormone via intron 1 and promoter containing a mutant TATA box (CATAAAA) has poor porcine β-casein gene activity.

Multispecies comparison of the casein gene loci and evolution of casein gene family

Caseins, the major milk proteins, are present in a genomic cluster spanning 250-350 kb. The divergence at the coding level between human, rodent, and cattle sequences is rather extensive for most of the genes in this region. Nevertheless, comparative analysis of genomic sequences harboring the casein gene cluster region of these species (with equal evolutionary distances 79-88 Myr) shows that the organization and orientation of the genes is highly conserved. The conserved gene structure indicates that the molecular diversity of the casein genes is achieved through variable use of exons in different species and high evolutionary divergence. Comparative analysis also revealed the presence within two species of uncharacterized casein family members and ruled out the previously held notion that another gene family, located in this region, is primate-specific. Several other new genes as well as conserved noncoding sequences with potential regulatory functions were identified. All genes identified in this region are, or are predicted to be, secreted proteins involved in mineral homeostasis, nutrition, and/or host defense, and are mostly expressed in the mammary and/or salivary glands. These observations suggest a possible common ancestry for the genes in this region.

Isolates of Microbotryum violaceum from North American host species are phylogenetically distinct from their European host-derived counterparts

Microbotryum violaceum is a basidiomycete that infects the anthers of its Caryophyllaceae host species. Individual fungal isolates are host limited, though they are not morphologically distinct. This study used variable regions of the highly conserved gamma-tubulin, beta-tubulin, and ribosomal RNA-encoding genes to determine the relationships among M. violaceum fungal isolates from different host species and different geographical locations. Phylogenetic trees from intron nucleotide sequences in two protein-coding genes were compared to trees produced from internal transcribed spacers (ITS) of rDNA. Phylogenetic analyses suggested that there are two clades, one from North America and one from Europe. Isolates from both clades grouped according to host species, although in some analyses isolates from closely related host species were placed together. These results are consistent with the view that M. violaceum has experienced cospeciation with its hosts.

A hybrid bovine beta-casein/bGH gene directs transgene expression to the lung and mammary gland of transgenic mice

We investigated spatial and temporal expression of bGH controlled by two different sizes (1.8 kb and 15 kb) of 5'-flanking sequences of the bovine beta-casein in transgenic mice. In the 1.8-kb promoter-containing mice, bGH expression was specifically confined to lung and mammary gland at lactation. While mammary gland expression was highly variable depending on the lines, lung expression was relatively constant with a high level in most lines. Moreover, this dual-tissue specificity of bGH expression was consistently retained in all of the 15 kb-promoter-containing mice, although a low ectopic expression was sometimes detected in salivary gland or brain. During mammary gland development in the 1.8-kb promoter-containing mice was mammary gland expression first detected at lactation, following the bovine rather than murine pattern of beta-casein expression. In contrast, lung expression was almost constant regardless of mammary gland developmental state or sex. Therefore, it can be concluded that a combination of the bovine beta-casein promoter and bGH gene directs a distinct dual-tissue specific bGH expression with different regulatory mechanisms between mammary gland and lung and as little as 1.8-kb promoter is sufficient for the proper regulation of the bovine beta-casein gene in mammary gland.

Constitutive Activation of the JAK2/STAT5 Signal Transduction Pathway Correlates With Growth Factor Independence of Megakaryocytic Leukemic Cell Lines

The factor-independent Dami/HEL and Meg-01 and factor-dependent Mo7e leukemic cell lines were used as models to investigate JAK/STAT signal transduction pathways in leukemic cell proliferation. Although Dami/HEL and Meg-01 cell proliferation in vitro was independent of and unresponsive to exogenous cytokines including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), and tumor necrosis factor- (TNF-), the growth of Mo7e cells was dependent on hematopoietic growth factors. When these cell lines were cultured in medium without cytokines, a constitutively activated STAT-like DNA-binding factor was detected in nuclear extracts from both Dami/HEL and Meg-01 cells. However, the STAT-like factor was not detectable in untreated Mo7e cells, but was activated transiently in Mo7e cells in response to cytokine treatments. The constitutively activated and cytokine-induced STAT-like DNA-binding factor in these three cell lines was identified as STAT5 by oligonucleotide competition gel mobility assays and by specific anti-STAT antibody gel supershift assays. Constitutive activation of JAK2 also was detected in the factor-independent cell lines, but not in Mo7e cells without cytokine exposure. Meg-01 cells express a p185 BCR/ABL oncogene, which may be responsible for the constitutive activation of STAT5. Dami/HEL cells do not express the BCR/ABL oncogene, but increased constitutive phosphorylation of Raf-1 oncoprotein was detected. In cytokine bioassays using growth factor-dependent Mo7e and TF-1 cells as targets, conditioned media from Dami/HEL and Meg-01 cells did not show stimulatory effects on cell proliferation. Our results indicate that the constitutive activation of JAK2/STAT5 correlates with the factor-independent growth of Dami/HEL and Meg-01 cells. The constitutive activation of JAK2/STAT5 in Dami/HEL cells is triggered by a mechanism other than autocrine cytokines or the BCR/ABL oncoprotein.

Constitutive Activation of the JAK2/STAT5 Signal Transduction Pathway Correlates With Growth Factor Independence of Megakaryocytic Leukemic Cell Lines

The factor-independent Dami/HEL and Meg-01 and factor-dependent Mo7e leukemic cell lines were used as models to investigate JAK/STAT signal transduction pathways in leukemic cell proliferation. Although Dami/HEL and Meg-01 cell proliferation in vitro was independent of and unresponsive to exogenous cytokines including granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), IL-6, thrombopoietin (TPO), and tumor necrosis factor- (TNF-), the growth of Mo7e cells was dependent on hematopoietic growth factors. When these cell lines were cultured in medium without cytokines, a constitutively activated STAT-like DNA-binding factor was detected in nuclear extracts from both Dami/HEL and Meg-01 cells. However, the STAT-like factor was not detectable in untreated Mo7e cells, but was activated transiently in Mo7e cells in response to cytokine treatments. The constitutively activated and cytokine-induced STAT-like DNA-binding factor in these three cell lines was identified as STAT5 by oligonucleotide competition gel mobility assays and by specific anti-STAT antibody gel supershift assays. Constitutive activation of JAK2 also was detected in the factor-independent cell lines, but not in Mo7e cells without cytokine exposure. Meg-01 cells express a p185 BCR/ABL oncogene, which may be responsible for the constitutive activation of STAT5. Dami/HEL cells do not express the BCR/ABL oncogene, but increased constitutive phosphorylation of Raf-1 oncoprotein was detected. In cytokine bioassays using growth factor-dependent Mo7e and TF-1 cells as targets, conditioned media from Dami/HEL and Meg-01 cells did not show stimulatory effects on cell proliferation. Our results indicate that the constitutive activation of JAK2/STAT5 correlates with the factor-independent growth of Dami/HEL and Meg-01 cells. The constitutive activation of JAK2/STAT5 in Dami/HEL cells is triggered by a mechanism other than autocrine cytokines or the BCR/ABL oncoprotein.

A distal enhancer region in the human beta-casein gene mediates the response to prolactin and glucocorticoid hormones

The 5' flanking region of the human beta-casein gene was investigated for the presence of regulatory sequences mediating the action of the lactogenic hormones prolactin and dexamethasone. DNA encompassing 9389 base pairs of the flanking region was isolated and a sequence comparison performed with regulatory regions previously identified in the beta-casein gene of rodents and ruminants. The analysis revealed the presence of a distal region between -4700 and -4550 with a high percentage of identity to the bovine beta-casein enhancer region, and a proximal region between -1 and -200 similar to the proximal promoter regions found in rodents and ruminants. Reporter gene constructs under the control of the distal or the proximal region of the human beta-casein gene were tested for their responsiveness to prolactin and dexamethasone. In transfection experiments, the distal region functioned as a lactogenic hormone inducible enhancer, whereas the proximal region exhibited low activity. In electromobility shift assays, multiple binding sites for Stat5, CCAAT/enhancer-binding proteins, and Ets domain proteins were identified in the distal human enhancer. These transcription factors have already been demonstrated as important regulators of the transcription of milk protein genes in rodents. Thus, a common set of transcription factors appears to be required for the expression of the human beta-casein gene and of milk protein genes in other species.

Pretreatment with glucocorticoid is essential for lactogenic induction of the bovine beta-casein/CAT expression in HC11 cells

Hormonal regulation of the bovine beta-casein gene expression was studied in a murine mammary epithelial HC11 cells and compared with that of the rat beta-casein gene expression. CAT expression vectors driven by their promoter sequences were transfected into HC11 cells. Stable transfectents were treated with lactogenic hormones, dexamethasone and prolactin for 2 days in confluent cultures. While the lactogenic hormones synergistically induced a strong activation of the rat beta-casein/CAT expression, neither a single or combined treatment of dexamethasone and prolactin induced the bovine beta-casein/CAT expression. To test a sequential treatment effect of lactogenic hormones on the bovine beta-casein/CAT expression, cells were first treated with either dexamethasone or prolactin for various days and then subjected to the second treatment with both hormones for 2 days. Only dexamethasone-, but not prolactin-pretreated cells showed a strong lactogenic induction. Moreover, the fold induction of dexamethasone-pretreated cells increased gradually as a function of duration of dexamethasone pretreatment. A series of the bovine beta-casein/CAT constructs with different length of the bovine beta-casein 5' flanking region ranged from 0.3 kb to about 15 kb was analyzed in 12-days dexamethasone-pretreated cultures. CAT expression was increased even in 0.3 kb-containing construct, but prominent induction was seen in more than 1.8 kb-containing constructs. Therefore, it could be concluded that a long-term dexamethasone pretreatment is essential for lactogenic induction of the bovine beta-casein expression and the 0.3 kb proximal promoter region is important, but more distal promoter element(s) is necessary for mediating the coordinated action of lactogenic hormones to the bovine beta-casein expression.

Hormonally regulated double- and single-stranded DNA-binding complexes involved in mouse beta-casein gene transcription

Transcription of the 252-base pair-long mouse beta-casein gene promoter is induced by the synergistic action of insulin, prolactin, and glucocorticoid in a primary mammary epithelial cell culture. The promoter contains a region termed block C having a highly conserved sequence and position among many casein genes. Mutation of block C reduced the response of the promoter to lactogenic hormones 84%. Nuclear extracts from lactating mouse mammary glands contained both a double-stranded and a single-stranded DNA binding protein complex (DS1 and SS), which specifically bind to the sequences AAATTAGCATGT and CCACAA of block C, respectively. The DS1 and the SS protein complexes were approximately 400 and 280 kDa, respectively. Each complex contained a DNA-binding component(s) having a molecular mass of approximately 120 kDa for DS1 and 80 and 65 kDa for SS. Deoxycholate, which interferes with the protein-protein interactions, inhibited the binding activities of DS1 and SS. The maximal increase in the binding activity of DS1 and SS in the mammary gland occurred during pregnancy and during lactation, respectively. In organ culture, the DS1 activity is increased by epidermal growth factor or prolactin in combination with insulin, whereas the SS activity is enhanced by insulin, prolactin, and glucocorticoid. These results suggest that multiprotein complexes binding to the double- and single-stranded DNA of block C mediate hormonal induction of beta-casein gene transcription.

Mammary gland expression of transgenes and the potential for altering the properties of milk

Transgenic animals are a useful in vivo experimental model for assessing the ability and impact of foreign gene expression in a biological system. Transgenic mice are most commonly used, while transgenic sheep, goats, pigs and cows have also been developed for specific, "applied" purposes. Most of the work directed at targeting expression of transgenes to the mammary gland of an animal, by using a milk gene promoter, has been with the intent of either studying promoter function or recovering the desired protein from the milk. Transgenic technology can also be used to alter the functional and physical properties of milk resulting in novel manufacturing properties. The properties of milk have been altered by adding a new protein with the aim of improving the milk, not of recovering the protein for other uses.

Computer-aided analysis of potential transcription-factor binding sites in the rabbit beta-casein gene promoter

Computer analysis of putative cis- and trans-regulatory sequences in the promoter region of the rabbit beta-casein gene is described. Nucleotide sequences up to 2096 bp upstream of the initiation site were compared to known consensus sequences of both ubiquitous and specific transcription factor motifs as well as to those described as characteristic for milk protein genes. Our analysis demonstrated that 5 independent motifs described for milk protein gene promoters exist. Four of them--'Groenen structure', 'Yu-Lee' 1 and 6 sequence and 'Oka box A'--were found in the beta-casein gene promoter; surprisingly, no typical milk box was found. More than one hundred and fifty putative binding motifs were found, representing 56 various consensus sequences. These sequences are located both inside and outside structures typical of milk protein gene promoters and include sequences homologous to mammary gland specific, hormone specific and ubiquitous transcription factors. Our analysis suggests that milk protein gene promoters can bind many transcription factors and assure complex regulation by hormonal and tissue specific factors.

Production of pharmaceutical proteins from transgenic animals

Different systems are being studied and used to prepare recombinant proteins for pharmaceutical use. The blood, and still more the milk, from transgenic animals appear a very attractive source of pharmaceuticals. The cells from these animals are expected to produce well-matured proteins in potentially huge amounts. Several problems remain before this process becomes used in a large scale. Gene transfer remains a difficult and costly task for farm animals. The vectors carrying the genes coding for the proteins of interest are of unpredictable efficiency. Improvement of these vectors includes the choice of efficient promoters, introns and transcription terminators, the addition of matrix attached regions (MAR) and specialized chromatin sequences (SCS) to enhance the expression of the transgenes and to insulate them from the chromatin environment. Mice are routinely used to evaluate the gene constructs to be transferred into larger animals. Mice can also be utilized to prepare amounts as high as a few hundred mg of recombinant proteins from their milk. Rabbit appears adequate for amounts not higher than 1 kg per year. For larger quantities, goat, sheep, pig and cow are required. No recombinant proteins extracted from the blood or milk of transgenic animals are yet on the market. The relatively slow but real progress to improving the efficiency of this process inclines to be reasonably optimistic. Predictive reports suggest that 10% of the recombinant proteins, corresponding to a 100 million dollars annual market, will be prepared from the milk of transgenic animals by the end of the century.

Expression of human lysozyme mRNA in the mammary gland of transgenic mice

Owing to its inherent antimicrobial effect and positive charge, the expression of human lysozyme in bovine milk could be beneficial by altering the overall microbial level and the functional and physical properties of the milk. We have used transgenic mice as model systems to evaluate the expression of human lysozyme containing fusion gene constructs in the mammary gland. Expression of human lysozyme was targeted to the mammary gland by using the 5' promoter elements of either the bovine beta (line B mice) or alpha s1 (line H mice) casein genes coupled to the cDNA for human lysozyme. Expression of human lysozyme mRNA was not found in mammary tissue from any of line B mice. Tissues were analysed from six lines of H mice and two, H6 and H5, were found to express human lysozyme mRNA in the mammary gland at 42% and 116%, respectively, of the levels of the endogenous mouse whey acidic protein gene. At peak lactation, female mice homozygous for the H5 and H6 transgene have approximately twice the amount of mRNA encoding human lysozyme as hemizygous animals. Expression levels of human lysozyme mRNA in the mammary gland at time points representing late pregnancy, early, peak and late lactation corresponded to the profile of casein gene expression. Human lysozyme mRNA expression was not observed in transgenic males, virgin females or in the kidney, liver, spleen or brain of lactating females. A very low level of expression of human lysozyme mRNA was observed in the salivary gland of line H5.

The gamma-tubulin-encoding gene from the basidiomycete fungus, Ustilago violacea, has a long 5'-untranslated region

The gene (gamma-tub) encoding gamma-tubulin (gamma-Tub) was isolated from a cosmid library constructed for Ustilago violacea by using a PCR-amplified DNA fragment as a probe. About 2.8 kb of DNA sequence was analyzed and found to encode a protein of 469 amino acids highly homologous to the gamma-Tub from other organisms. There were eight introns interrupting the coding sequence. A 'TATA'-like sequence was found 389 bp upstream from the initial Met codon. No polyadenylation signal was found in the 3' non-coding region. Southern blot analyses indicated that gamma-tub is a single-copy gene. Northern blot analyses indicated that a 1.81-kb RNA species was transcribed. Primer extension experiments determined that the transcription start point (tsp) is at 58 bp downstream from the putative TATA box, with another possible tsp at 95 bp downstream. The long 5' non-coding sequence of the RNA contained several small open reading frames; their possible roles in the regulation of gamma-tub translation are discussed.

Multiple octamer binding sites in the promoter region of the bovine alpha s2-casein gene

Using a set of overlapping oligonucleotides from the promoter region of the bovine alpha s2-casein gene we have identified two nuclear factors which probably are involved in expression of this gene and the related calcium sensitive alpha s1- and beta-casein genes. One of these factors which was present in extracts of all tissues that have been tested including Hela cells turned out to be the octamer binding protein OCT-1. Oct-1 binds with different affinity to 4 sites at positions centred around -480, -260, -210 and -50. The strongest of these 4 binding sites, the one around position -50, is highly conserved in all calcium sensitive caseins of mouse, rat, rabbit and cattle. The other nuclear factor (MGF, mammary gland factor) which is specifically expressed in the mammary gland, binds to a site around position -90. This binding site is also highly conserved in all calcium sensitive caseins of mouse, rat, rabbit and cattle.

The prospects for domesticating milk protein genes

It is possible to convert milk glands of transgenic animals into bioreactors producing heterologous proteins such as scarce human pharmaceuticals. To predictably and successfully engineer the milk gland, we will need a thorough understanding of its physiology. Expression studies in transgenic animals have located mammary specific and hormone inducible transcription elements in the promoter/upstream regions of milk protein genes, and transfection studies in cell lines or primary cells have identified constitutive and hormone inducible elements. Most importantly, it appears that in addition to individual promoter based transcription elements structural features of milk protein chromosomal loci may contribute to the tight developmental and hormonal regulation. I will discuss milk protein gene regulation with emphasis on regulatory differences between genes and species, and the possibility that transcription elements function only properly within genetically defined chromatin domains. Novel strategies to build mammary expression vectors and to test their functionality without pursuing the standard transgenic route will be presented. Finally, I will discuss homologous recombination with the goal to target milk protein genes. Only through the domestication of milk protein genes will we be able to use their full potential in the mammary bioreactor.

Structure of the gene encoding rabbit alpha s1-casein

The complete nucleotide sequence of the entire rabbit alpha s1-casein-encoding gene Aslca and its flanking regions was determined. These data represent the first complete primary sequence of an Aslca gene. The gene consists of 19 exons spread over 16 kb. Highly conserved sequences were found between this gene and other casein-encoding genes mainly upstream from the gene from position -180 to -10. Several repeated interspersed elements of unknown function were also identified within introns.

Developmental and environmental regulation of a mammary gland-specific nuclear factor essential for transcription of the gene encoding beta-casein

During the lactation period, mammary epithelial cells secrete large amounts of milk proteins. The coordinate regulation of milk protein expression is effected by the lactogenic hormones. We have investigated the activity of a mammary gland-specific transcription factor (MGF), which mediates hormonal influences at the level of a milk protein gene promoter. MGF-binding sites are present in the promoters of the most abundantly expressed milk protein genes. Mutation of the MGF-binding site in the beta-casein gene promoter completely abolishes responsiveness of the promoter to lactogenic hormones in cultured mammary epithelial cells. MGF activity is closely controlled in vivo. High MGF levels were found in mouse mammary gland nuclear extracts toward the end of pregnancy and during lactation. Withdrawal of suckling pups from their mothers during the lactation period caused a strong and rapid decrease of MGF activity. Readdition of pups to their mothers restored maximal MGF levels within 4 hr. We investigated MGF phosphorylation as a possible posttranslational modification responsible for regulation of the DNA-binding activity of MGF. Treatment of nuclear extracts from lactating mammary glands with potato acid phosphatase abolished MGF-binding activity. Casein kinase II phosphorylation of nuclear extracts from animals withdrawn from their pups for 24 hr enhanced MGF-binding activity. These results suggest that the reversible activation of MGF by suckling and withdrawal might be mediated by the action of kinases and phosphatases.

Genomic organization of the bovine alpha-S1 casein gene

We report the sequence of the complete bovine alpha-s1 casein gene eludicating for the first time the genomic organization of an alpha-s type casein gene. Extending over 17508 bp the gene is split into 19 exons, ranging in size from 24 bp to 385 bp. Except for the translational stop codon not a single coding triplet of the alpha-s1 reading frame is disrupted by any of the splice junctions, which all confirm to known splice consensus sequences. Nine out of 16 coding exons begin with a 'GAX' codon, specific for glutamate. Splicing of this codon from exon 10 to the preceding exon creates a major phosphorylation site. An intron-exon-intron stretch of 154 bp comprising exons 10 and 13 is found precisely duplicated. Associated with the gene, copies of 8 atriodactyla retroposons are found, 6 of which are interspersed into the sequences of the three longest introns. We discuss the possibility that three functional parts of the gene have been recruited and evolutionary conserved at a time before gene diversification gave rise to the separate evolution of alpha- and beta-type casein-genes.

Beta-casein gene promoter activity is regulated by the hormone-mediated relief of transcriptional repression and a mammary-gland-specific nuclear factor

Transcription from the beta-casein milk protein gene promoter is induced by the synergistic action of glucocorticoid and prolactin hormones in the murine mammary epithelial cell line, HC11. We analyzed the binding of nuclear proteins to the promoter and determined their binding sites. Site-directed mutagenesis was used to determine the function of nuclear factor binding. During lactogenic hormone induction of HC11 cells, the binding of two nuclear factors increased. The binding of two other nuclear factors, present in uninduced cells, decreased. The basal activity of the promoter could be increased to and above the level of the induced wild-type promoter when the recognition sequences of the negatively regulated factors were mutated. This suggests that the beta-casein promoter is regulated by the relief of the repression of transcription. An essential tissue-specific factor was also found in nuclear extracts from the mammary glands of mice. Mutation of its recognition sequence in the beta-casein promoter led to the abolition of the induction of transcription by lactogenic hormones. The DNA sequences recognized by all five of these nuclear factors are conserved in the promoters of different casein genes from several species, confirming their importance in the regulation of milk protein gene transcription.

Beta-casein gene promoter activity is regulated by the hormone-mediated relief of transcriptional repression and a mammary-gland-specific nuclear factor

Transcription from the beta-casein milk protein gene promoter is induced by the synergistic action of glucocorticoid and prolactin hormones in the murine mammary epithelial cell line, HC11. We analyzed the binding of nuclear proteins to the promoter and determined their binding sites. Site-directed mutagenesis was used to determine the function of nuclear factor binding. During lactogenic hormone induction of HC11 cells, the binding of two nuclear factors increased. The binding of two other nuclear factors, present in uninduced cells, decreased. The basal activity of the promoter could be increased to and above the level of the induced wild-type promoter when the recognition sequences of the negatively regulated factors were mutated. This suggests that the beta-casein promoter is regulated by the relief of the repression of transcription. An essential tissue-specific factor was also found in nuclear extracts from the mammary glands of mice. Mutation of its recognition sequence in the beta-casein promoter led to the abolition of the induction of transcription by lactogenic hormones. The DNA sequences recognized by all five of these nuclear factors are conserved in the promoters of different casein genes from several species, confirming their importance in the regulation of milk protein gene transcription.

Structure of the gene encoding rabbit beta-casein

The entire rabbit beta-casein-encoding gene and 400 bp upstream were sequenced. Eight introns, located essentially at a position similar to the corresponding gene in other species, were found. Strong homology with several casein-encoding genes from rabbit and from other species was observed in the upstream region of the gene. Repeated sequences of unknown function were also located within introns.

Long range restriction analysis of the bovine casein genes

Pulsed field gel electrophoresis (PFGE) was used to analyse the organization of the bovine alpha s1, alpha s2, beta and kappa casein genes. High molecular weight DNA was prepared from fibroblasts and lymphocytes embedded in agarose and was digested with the restriction endonucleases Clal, Sall, Smal, Xhol. The digestion products were separated by PFGE, transfered to nitrocellulose filters and hybridized to probes corresponding to the cDNAs of the four bovine casein genes. The casein genes were demonstrated to be physically linked within a region of 300 kb, represented by two adjacent Xhol fragments in fibroblasts and by a single fragment in lymphocytes. A restriction map of the casein locus was derived and the order of the genes was shown to be kappa, alpha s2, beta, alpha s1.

Extracellular matrix and hormones transcriptionally regulate bovine beta-casein 5' sequences in stably transfected mouse mammary cells

Milk protein regulation involves synergistic action of lactogenic hormones and extracellular matrix (ECM). It is well established that substratum has a dramatic effect on morphology and function of mammary cells. The molecular mechanisms that regulate the ECM- and hormone-dependent gene expression, however, have not been resolved. To address this question, a subpopulation (designated CID 9) of the mouse mammary epithelial cell strain COMMA-1D has been developed in which more than 35% of the cells express beta-casein, form alveoli-like structures when plated onto a reconstituted basement membrane, and secrete beta-casein unidirectionally into a lumen. These cells were stably transfected with a series of chloramphenicol acetyltransferase (CAT) fusion genes to study transcriptional regulation of the bovine beta-casein gene. The expression of CAT in these lines demonstrated a striking matrix and hormone dependency (greater than 150-fold induction in some cases). This regulation occurred primarily at the transcriptional level and was dependent on the length of the 5' flanking region of the beta-casein promotor. Both matrix and hormonal control of transcription occurred within at least the first 1790 base pairs upstream and/or 42 base pairs downstream of the transcriptional initiation site. The ECM effect was independent of glucocorticoid stimulation. However, prolactin was essential and hydrocortisone further increased CAT expression. Endogenous beta-casein expression in these lines was similar to that of the parent CID 9 cells. Our data indicate the existence of matrix-dependent elements that regulate transcription.

Transfection of beta-casein chimeric gene and hormonal induction of its expression in primary murine mammary epithelial cells

To study the regulatory sequence elements responsible for casein gene expression, we constructed a chimeric gene containing 5.3 kilobases (kb) of the 5'-flanking sequence and 1.6 kb of the 3'-flanking sequence of the mouse beta-casein gene fused to the bacterial chloramphenicol acetyl-transferase (CAT) gene. The chimeric gene was transfected by the calcium phosphate-precipitation procedure into primary mouse mammary epithelial cells prepared from pregnant mice. The transfection procedure had negligible effect on expression of the endogenous beta-casein gene. Expression of the beta-casein-CAT chimeric gene required the synergistic actions of insulin, hydrocortisone, and prolactin. Expression of the chimeric gene also depended on the appropriate substratum because the degree of hormonal induction of the chimeric gene was much higher in cells cultured on a reconstituted basement membrane (Matrigel) than in cells cultured on either type I collagen gel or plastic. On the other hand, the expression of a simian virus 40-CAT chimeric gene in which the CAT gene was driven by the early promoter of the virus was not influenced by the hormonal milieu and occurred at the highest level in cells cultured on plastic. Additional transfection experiments with a series of beta-casein-CAT constructs suggested the existence of regulatory elements responsible for hormonal induction and negative regulatory elements.

Rabbit β-Casein Promoter Directs Secretion of Human Interleukin-2 into the Milk of Transgenic Rabbits

To test the potential usefulness of transgenic rabbits as production systems for human proteins of pharmaceutical value, we cloned the rabbit beta-casein promoter and fused it to the genomic sequence of the human interleukin-2 (hIL2) gene. Four transgenic female rabbits were tested for expression and biological activity of the foreign protein in their milk. The milk of all four females proved to contain biologically active hIL2. The results show that transgenic rabbits may represent a convenient and economic system for the rapid production of biologically active protein in milk.

Efficient tissue-specific expression of bovine alpha-lactalbumin in transgenic mice

Transgenic mice carrying a 3.2-kb genomic fragment comprising the bovine alpha-lactalbumin gene transcription unit with 750 bp and 336 bp of 5' and 3' flanking regions, respectively, were generated. Tissue-specific expression of the transgene was observed in five out of the six lines analyzed at levels close to the endogenous gene in two of them.

Isolation and characterization of the bovine kappa-casein gene

1. The bovine kappa-casein gene has been isolated as a series of overlapping lambda clones and shown to consist of five exons distributed over a total length of approximately 13 kb. Most of the mature protein-coding sequence is contained in a single large exon. 2. Approximately 65% of the gene has been sequenced together with portions of the 5'- and 3'-flanking sequences. The immediate 5'-flanking sequence contains several motifs which are characteristic of upstream regions including a TATA box, a CAAT box, a sequence similar to that recognized by transcription factor AP-1 and a purine-rich sequence resembling that found upstream in all other lactoprotein genes. Other possible regulatory sequences are found upstream of exon 4. 3. The organization of the kappa-casein gene, together with its upstream sequence, confirms previous conclusions that it is unrelated to the calcium-sensitive-casein gene family to which it is linked. Evidence is presented which supports a previous suggestion that kappa-casein and the fibrinogens are evolutionarily related. 4. Intron sequences contain several examples of the A family of the artiodactyl Alu-like repeated sequences, together with a single example of a C-family sequence. The remainders of the introns of the kappa-casein gene, compared with the repeat elements and exons, are A + T-rich. 5. Among the lambda clones isolated, representatives were found of the A and B genetic variants which can be distinguished by restriction-enzyme analysis. Several other examples of polymorphisms in the non-coding region were found.