The cDNA for the beta-subunit of human chorionic gonadotropin suggests evolution of a gene by readthrough into the 3'-untranslated region

Chromosome assignment of genes encoding the alpha and beta subunits of glycoprotein hormones in man and mouse

The chromosomal locations of the genes for the common alpha subunit of the glycoprotein hormones and the beta subunit of chorionic gonadotropin in humans and mice have been determined by restriction enzyme analysis of DNA isolated from somatic cell hybrids. The CG alpha gene (CGA), detected as a 15-kb BamHI fragment in human DNA by hybridization to CG alpha cDNA, segregated with the chromosome 6 enzyme markers ME1 (malic enzyme, soluble) and SOD2 (superoxide dismutase, mitchondrial) and an intact chromosome 6 in human-rodent hybrids. Cell hybrids containing portions of chromosome 6 allowed the localization of CGA to the q12 leads to q21 region. The greater than 30- and 6.5-kb BamHI CGB fragments hybridizing to human CG beta cDNA segregated concordantly with the human chromosome 19 marker enzymes PEPD (peptidase D) and GPI (glucose phosphate isomerase) and a normal chromosome 19 in karyotyped hybrids. A KpnI-HindIII digest of cell hybrid DNAs indicated that the multiple copies of the CG beta gene are all located on human chromosome 19. In the mouse, the alpha subunit gene, detected by a mouse thyrotropin (TSH) alpha subunit probe, and the CG beta-like sequences (CG beta-LH beta), detected by the human CG beta cDNA probe, are on chromosomes 4 and 7, respectively.

The chorionic gonadotropin alpha-subunit gene is on human chromosome 18 in JEG cells

The gene for the alpha subunit of human chorionic gonadotropin (hCG) has been tentatively assigned to human chromosome 18. This localization was accomplished through the use of Southern blot analysis. A full-length cDNA probe for the hCG alpha subunit and DNA isolated from a series of somatic hybrids between mouse and human cells were utilized to make this assignment. In addition, in situ hybridization with normal human peripheral blood lymphocytes as a source of human chromosomes and with the same cDNA probe confirmed this result. The presence of human chromosome 18 was required for the detection of DNA fragments characteristic of the alpha-hCG gene. These results are consistent with our previous observation that human chromosomes 10 and 18 are required for the production of hCG in cultured cells.

Cytological distribution of chorionic gonadotropin subunit and placental lactogen messenger RNA in neoplasms derived from human placenta

Normal trophoblast of the human placenta elaborates at least two major protein hormones, chorionic gonadotropin (hCG), and placental lactogen (hPL). There are several gestational trophoblastic diseases of the placenta called hydatidiform mole, invasive mole, and choriocarcinoma. Molar and choriocarcinoma tissues characteristically synthesize large amounts of hCG and small quantities of hPL. To examine the role of trophoblast differentiation in the expression of the hCG and hPL genes, we studied the cytological distribution of their messenger RNA (mRNA) in tissue sections of human hydatidiform mole and choriocarcinoma by in situ hybridization. Histologically, these tissues are in different stages of cellular differentiation. In normal placenta, hCG alpha and -beta mRNA can be localized to some cytotrophoblasts and primarily to the syncytium, whereas hPL mRNA appears only in the syncytial layer. In hydatidiform mole, which still retains placental villous morphology, the hPL gene and hCG alpha and -beta genes are expressed but are poorly localized because of the admixture of cyto- and syncytiotrophoblasts. By contrast, choriocarcinoma, which is devoid of placental villous pattern but in which the cyto- and syncytiotrophoblast-like components are distinguishable, expresses hCG alpha and -beta in the syncytial-like areas but little, if any, hPL. These results suggest that a certain level of trophoblast differentiation, such as villous formation, is associated with hPL expression, while the hCG alpha gene and the hCG beta gene can be expressed in more disorganized tissues that contain cytotrophoblastic elements.

Nucleotide sequence of the cDNA encoding the precursor of the beta subunit of rat lutropin

We have determined the nucleotide sequences of cDNAs encoding the precursor of the beta subunit of rat lutropin, a polypeptide hormone that regulates gonadal function, including the development of gametes and the production of steroid sex hormones. The cDNAs were prepared from poly(A)+ RNA derived from the pituitary glands of rats 4 weeks after ovariectomy and were cloned in bacterial plasmids. Bacterial colonies containing transfected plasmids were screened by hybridization with a 32P-labeled cDNA encoding the beta subunit of human chorionic gonadotropin, a protein that is related in structure to lutropin. Several recombinant plasmids were detected that by nucleotide sequence analyses contained coding sequences for the precursor of the beta subunit of lutropin. Complete determination of the nucleotide sequences of these cDNAs, as well as of cDNA reverse-transcribed from pituitary poly(A)+ RNA by using a synthetic pentadecanucleotide as a primer of RNA, provided the entire 141-codon sequence of the precursor of the beta subunit of rat lutropin. The precursor consists of a 20 amino acid leader (signal) peptide and an apoprotein of 121 amino acids. The amino acid sequence of the rat lutropin beta subunit shows similarity to the beta subunits of the ovine/bovine, porcine, and human lutropins (81, 86, and 74% of amino acids identical, respectively). Blot hybridization of pituitary RNAs separated by electrophoresis on agarose gels showed that the mRNA encoding the lutropin beta subunit consists of approximately 700 bases. The availability of cDNAs for both the alpha and beta subunits of lutropin will facilitate studies of the regulation of lutropin expression.

Cloning of cDNA encoding the pre-beta subunit of mouse thyrotropin

Double-stranded cDNA was synthesized from sucrose gradient-purified poly(A)+ mRNA from a mouse thyrotropic tumor, inserted into the Pst I site of plasmid pBR322 by using poly(dC) . poly(dG) homopolymeric extensions, and cloned in Escherichia coli RRI. Plasmids containing cDNA sequences coding for the beta subunit of thyrotropin (TSH) were identified by cell-free translation of hybrid-selected mRNA and immunoprecipitation with specific antibody to TSH beta subunit. Determination of the nucleotide sequence of one cDNA, 595 base pairs in length, allowed us to deduce the entire amino acid sequence of the mouse TSH beta subunit. The pre-beta subunit contains a 20-amino acid amino-terminal signal sequence followed by a 118-amino acid mature TSH beta subunit. There is 85-90% homology in amino acid sequence between mouse TSH beta subunit and subunits from man, pig, and cow; however, the mouse subunit contains an additional 5 or 6 amino acids at its carboxyl terminus compared to the bovine or human and pig subunits, respectively. TSH beta-subunit mRNA from mouse thyrotropic tumor was estimated to be 750 nucleotides in length by hybridization with labeled TSH beta-subunit cDNA.

Expression of the human interferon-gamma cDNA in yeast

Several plasmids which direct the expression of human interferon-gamma (IFN-gamma) cDNA in the yeast Saccharomyces cerevisiae under the transcriptional control of the 3-phosphoglycerate kinase (PGK) promoter have been constructed. Up to 25 x 10(6) units per liter culture of biologically active IFN-gamma are synthesized in yeast transformed with these expression plasmids. The amount of IFN-gamma mRNA in yeast transformed with the IFN-gamma expression plasmids is much lower than the amount of PGK mRNA present when the PGK gene and promoter are inserted on a similar plasmid. The major and three minor sites of transcription initiation have been localized. The major starting point of the IFN-gamma mRNA is at 4 basepairs upstream of the PGK mRNA start. All transcription starts map at an ApG as does PGK mRNA. The yeast IFN-gamma mRNA terminates in a specific segment of the 3' untranslated region of the IFN-gamma cDNA. The polyadenylation site at about 182-184 nucleotides 3' of the IFN-gamma stop codon is preceded by a sequence which is very similar to a sequence proposed to be involved in transcription termination and polyadenylation (49).

Effect of transfer RNA from various sources on placental messenger RNA translation

Poly(A+)-containing mRNA from human term placenta was used to direct protein synthesis in a nuclease-treated rabbit reticulocyte lysate, which is dependent on mRNA and tRNA for maximal activity. The major protein product was human pre-placental lactogen (hPL). Addition of tRNA from rabbit liver, rabbit reticulocyte, human first trimester and term placenta, human liver and yeast resulted in 2-5-fold stimulation of [35S]methionine incorporation into total protein. Although all mammalian tRNA increased hPL synthesis, the relative synthesis as compared to endogenous globin was markedly different and most efficient with tRNA from term placenta. Addition of yeast tRNA increased total incorporation 3-fold but decreased incorporation of [35S]methionine into pre-hPL. These results suggest that the population of isoacceptor tRNAs may influence the expression of hPL in term placenta. Results are discussed by showing codon bias and usage of mRNA coding for hPL, alpha- and beta-hCG, rabbit globin and yeast alcohol dehydrogenase I.

The cDNA sequence of a human epidermal keratin: divergence of sequence but conservation of structure among intermediate filament proteins

We have determined the DNA sequence of a cloned cDNA that is complementary to the mRNA for the 50 kilodalton (kd) human epidermal keratin. This provides the first amino acid sequence for a cytoskeletal keratin. Comparison of this sequence with those of other keratins reveals an evolutionary relationship between the cytoskeletal and the microfibrillar keratins, but shows no homology to matrix or feather keratins. The 50 kd keratin shares 28%-30% homology with partial sequences of other intermediate filament proteins, which suggests that keratins may be the most distantly related members of this class of fibrous proteins. Our computer analyses predict that the 50 kd keratin contains two long alpha-helical domains separated by a cluster of helix-inhibitory residues in the middle of the protein. These findings indicate that despite major sequence divergence among intermediate filament proteins, they retain sequences compatible with secondary structural features that appear to be common to all of them.

Genetic diseases: diagnosis by restriction endonuclease analysis

We have summarized a number of different genetic disorders which can be diagnosed at the DNA level using restriction endonuclease fragment analysis. A whole spectrum of defects can be recognized: point mutations, deletions, additions, and crossing-over products or hybrid genes. These same restriction endonuclease techniques can enable different genes to be marked by polymorphism patterns. Thus, abnormal genes can be identified even if their exact DNA lesion is unknown or cannot be directly detected. The progress that has been made with the hemoglobinopathies and the experience from this group of single gene disorders should find application to other diseases as soon as specific probes become available.

Possible role of flanking nucleotides in recognition of the AUG initiator codon by eukaryotic ribosomes

Sequences flanking the initiator codon in eukaryotic mRNAs are not random. Out of 153 messages examined, 151 have either a purine in position -3, or a G in position +4, or both. Thus, [A/G]XXAUGG emerges as the favored sequence for eukaryotic initiation sites. Nucleotides flanking nonfunctional AUG triplets, which occur in the 5'-noncoding region of a few eukaryotic messages, are different from those found at most functional sites. Whereas most authentic initiator codons are preceded by a purine (usually A) in position -3, most nonfunctional AUGs have a pyrimidine in that position. The observed asymmetry suggests that purines in positions -3 and +4 might facilitate recognition of the AUG condon during formation of initiation complexes. To test this idea, in vitro binding studies were carried out with 32P-labeled oligonucleotides. Binding of AUG-containing oligonucleotides to wheat germ ribosomes was significantly enhanced by placing a purine in position -3 or +4. The scanning model, which postulates that 40S ribosomal subunits attach at the 5'-end of a message and migrate down to the AUG codon, is discussed in light of these new observations. A modified version of the scanning mechanism is proposed.

Homology between the primary structure of alpha-fetoprotein, deduced from a complete cDNA sequence, and serum albumin

The entire DNA complementary to murine alpha-fetoprotein mRNA has been cloned in the plasmid vector pBR322 and its complete nucleotide sequence determined. The deduced amino acid sequence identifies a hydrophobic prepeptide of 19 amino acids and the complete primary structure of alpha-fetoprotein. Its structure reveals extensive homology to serum albumin and suggests that there are 15 disulphide bridges in the alpha-fetoprotein molecule, all at the same positions as those found in albumin. The AFP molecule can be organized into a three-domain structure almost identical to that of serum albumin, suggesting a common ancestral origin of the two proteins.