Heterogeneity at the 5' termini of mouse dihydrofolate reductase mRNAs. Evidence for multiple promoter regions

Transcript initiation, polyadenylation, and functional promoter mapping for the dihydrofolate reductase-thymidylate synthase gene of Toxoplasma gondii

The fused dihydrofolate reductase/thymidylate synthase gene of Toxoplasma gondii contains ten exons spanning approximately 8 kb of genomic DNA. We have examined the ends of DHFR-TS transcripts within this gene, and find a complex pattern including two discrete 5' termini and multiple polyadenylation sites. No TATAA box or other classical promoter motif is evident in 1.4 kb of genomic DNA upstream of the coding region, but transcript mapping by RNase protection and primer extension reveals two prominent 5' ends at positions -369 and -341 nt relative to the ATG initiation codon. Upstream genomic sequences include GC-rich regions and the (opposite strand) WGAGACG motif previously identified in other T. gondii promoters. Mutagenesis of recombinant reporter plasmids demonstrates that this region is essential for efficient transgene expression. Sequencing the 3' ends from multiple independent mRNA clones demonstrates numerous polyadenylation sites, distributed over >650 nt of genomic sequence beginning approximately 250 nt downstream of the stop codon. Within this region, certain sites seem to be preferred: 14 different positions were found among the 32 polyadenylated transcripts examined, but approximately 40% of the transcripts map to two loci. The 3' noncoding region is rich in A and T nucleotides, and contains an imperfect 50 nt direct repeat, but no obvious poly(A) addition signal was identified.

The murine G+C-rich promoter binding protein mGPBP is required for promoter-specific transcription

The archetypal TATA-box deficient G+C-rich promoter of the murine adenosine deaminase gene (Ada) requires a 48-bp minimal self-sufficient promoter element (MSPE) for function. This MSPE was used to isolate a novel full-length cDNA clone that encodes a 66-kDa murine G+C-rich promoter binding protein (mGPBP). The mGPBP mRNAs are ubiquitously expressed as either 3.0- or 3.5-kb forms differing in 3' polyadenylation site usage. Purified recombinant mGPBP, in the absence of any other mammalian cofactors, binds specifically to both the murine Ada gene promoter's MSPE and the nonhomologous human Topo IIalpha gene's G+C-rich promoter. In situ binding assays, immunoprecipitation, and Western blot analyses demonstrated that mGPBP is a nuclear factor that can form complexes with TATA-binding protein, TFIIB, TFIIF, RNA polymerase II, and P300/CBP both in vitro and in intact cells. In cotransfection assays, increased mGPBP expression transactivated the murine Ada gene's promoter. Sequestering of GPBP present in HeLa cell nuclear extract by immunoabsorption completely and reversibly suppressed extract-dependent in vitro transcription from the murine Ada gene's G+C-rich promoter. However, transcription from the human Topo IIalpha gene's TATA box-containing G+C-rich promoter was only partially suppressed and the adenovirus major late gene's classical TATA box-dependent promoter is totally unaffected under identical assay conditions. These results implicate GPBP as a requisite G+C-rich promoter-specific transcription factor and provide a mechanistic basis for distinguishing transcription initiated at a TATA box-deficient G+C-rich promoter from that initiated at a TATA box-dependent promoter.

The mouse p44 mitogen-activated protein kinase (extracellular signal-regulated kinase 1) gene. Genomic organization and structure of the 5'-flanking regulatory region

Mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase are ubiquitous kinases conserved from fungi to mammals. Their activity is regulated by phosphorylation on both threonine and tyrosine, and they play a crucial role in the regulation of proliferation and differentiation. We report here the cloning of the murine p44 MAP kinase (extracellular signal-regulated kinase 1) gene, the determination of its intron/exon boundaries, and the characterization of its promoter. The gene spans approximately eight kilobases (kb) and can be divided into nine exons and eight introns, each coding region exon containing from one to three of the highly conserved protein kinase domains. Primer extension analysis reveals the existence of two major start sites of transcription located at -183 and -186 base pairs (bp) as well as four discrete start sites for transcription located at -178, -192, -273, and -292 bp of the initiation of translation. However, the start site region lacks TATA-like sequences but does contain initiator-like sequences proximal to the major start sites obtained by primer extension. 1 kb of the promoter region has been sequenced. It contains three putative TATA boxes far upstream of the main start sites region, one AP-1 box, one AP-2 box, one Malt box, one GAGA box, one half serum-responsive element, and putative binding sites for Sp1 (five), GC-rich binding factor (five), CTF-NF1 (one), Myb (one), p53 (two), Ets-1 (one), NF-IL6 (two), MyoD (two), Zeste (one), and hepatocyte nuclear factor-5 (one). To determine the sites critical for the function of the p44 MAPK promoter, we constructed a series of chimeric genes containing variable regions of the 5'-flanking sequence of p44 MAPK gene and the coding region for luciferase. Activity of the promoter, measured by its capacity to direct expression of a luciferase reporter gene, is strong, being comparable with the activity of the Rous sarcoma virus promoter. Progressive deletions of the approximately 1 kb (-1200/-78) promoter region allowed us to define a minimal region of 186 bp (-284/-78) that has maximal promoter activity. Within this context, deletion of the AP-2 binding site reduces by 30-40% the activity of the promoter. Further deletion of this minimal promoter that removes the major start sites (-167/-78) surprisingly preserves promoter activity. This result implicates a major role of this region that contains the Sp1 sites.(ABSTRACT TRUNCATED AT 400 WORDS)

An inverted repeat motif stabilizes binding of E2F and enhances transcription of the dihydrofolate reductase gene

An overlapping inverted repeat sequence that binds the eukaryotic transcription factor E2F is 100% conserved near the major transcription start sites in the promoters of three mammalian genes encoding dihydrofolate reductase, and is also found in the promoters of several other important cellular and viral genes. This element, 5'-TTTCGCGCCAAA-3', is comprised of two overlapping, oppositely oriented sites which match the consensus E2F site (5'-TTT(C/G)(C/G)CGC-3'). Recent work has shown that E2F binding activity is composed of at least six related cellular polypeptides which are capable of forming DNA-binding homo- and heterodimers. We have investigated the binding of cellular E2F activity and of homo- and heterodimers of cloned E2F proteins to the inverted repeat E2F element. We have demonstrated that mutations in this element that abolish its inverted repeat nature, while preserving a single consensus E2F site, significantly decrease the binding stability of all of the forms of E2F tested. The rate of association of E2F-1/DP-1 heterodimers with the inverted repeat wild type site was not significantly different from those with the two single site mutated probes. Furthermore, the mutations decrease in vitro transcription and transient reporter gene expression 2-5-fold, an effect equivalent to that of abolishing E2F binding altogether. These data suggest a functional role that may explain the conservation of inverted repeat E2F elements among the DHFR promoters and several other cellular and viral promoters.

Characterization of the mouse Rep-3 gene: sequence similarities to bacterial and yeast mismatch-repair proteins

The mouse Rep-3 gene is transcribed divergently from the same promoter region as the dihydrofolate reductase-encoding gene and has a deduced amino-acid sequence that shares identity with the bacterial protein, MutS, which is involved in DNA mismatch repair. We have cloned Rep-3, mapped it and sequenced all of the known exons and their intron junction sequences. We find that the open reading frame is considerably larger than initially reported and that the most abundant form of Rep-3 mRNA encodes a protein of 123 kDa. The gene spans at least 134 kb and consists of 26 exons, including several alternatively spliced exons. All of the exon/intron junctions match the expected consensus sequences with the exception of the splice junctions for intron 6, which has AT and AC dinucleotides instead of the usual GT and AG bordering the exon sequences. The junction sequences for this intron share consensus sequences with three intron sequences from other genes, thereby helping to establish an alternative consensus sequence.

Characterization of the P4 promoter region of the human insulin-like growth factor II gene

The human insulin-like growth factor II (IGF-II) gene contains four promoters (P1, P2, P3 and P4). In order to determine the mechanism by which the P4 promoter is controlled, the human IGF-II P4 promoter was analyzed in cell lines. DNA sequence analysis of the human IGF-II P4 promoter gene showed that the P4 promoter region contains a TATA-like sequence and several G+C rich regions which are essential for transcription. Analysis of the transcription initiation site by S1 nuclease mapping revealed two transcription start sites; both are located immediately behind TATA-like sequence. To determine the location of sites that may be important for the function of the human IGF-II P4 promoter, we constructed chimeric genes of the human IGF-II P4 promoter fused to the coding region for chloramphenicol acetyltransferase (CAT). These constructs were transfected into HepG2, PLC/PRF/5, G401 and A549 cells, and were examined for CAT activity. All transfected cells showed a similar profile of CAT activity. Sequences responsible for putative enhancer and silencer regions were identified and the 5' flanking sequences of the human IGF-II P4 promoter contain negative regulatory regions (-213 to -174). The 53-base pair fragment located between 111 and 59 base pairs upstream of the start site contains positive regulatory activity. Gel mobility shift assay showed that Sp1 and another proteins might be involved in positive regulation of the human IGF-II P4 promoter.

Transcripts from a mosquito dihydrofolate reductase gene: evidence for heterogeneity at the 5' end

Heterogeneity among transcripts from the mosquito (Aedes albopictus) dihydrofolate reductase (DHFR) gene in wild-type C7-12 cells and in methotrexate-resistant Mtx-5011-128 cells has been analyzed by Northern blotting, RNAase mapping, and primer extension. In both sensitive and resistant cells, a major transcription initiation site mapped c. 11 nucleotides downstream of the TATAA box, near position -66 relative to the AUG codon. Two other major transcription initiation sites mapped approx. eight and 45 nucleotides, respectively, upstream of the TATAA box. In addition, at least six minor sites were detected, four of which mapped within TATA-like sequences. Within the AT-rich region flanking the 5'-end of the mosquito DHFR gene were four T-rich motifs and three "GTTTGTG" repeats. Additional "GTTTGTG" repeats occurred in the first exon and in the single 56 nucleotide intron of the mosquito DHFR gene. In contrast to the heterogeneity at the 5'-ends of mosquito DHFR transcripts, the 3'-end terminated at a single position, c. 22 nucleotides downstream of the polyadenylation signal.

Activation and suppression of a cryptic promoter in the intron of the human melanoma-associated ME491 antigen gene

A deletion mutant of the human melanoma-associated ME491 antigen gene starting at the first intron (lambda R31) differentially mediates the antigen expression depending on the cell type. Cryptic promoter activity residing in a 270-base-pair (bp) fragment of the first intron was examined by primer extension analysis and recombinant chloramphenicol acetyltransferase (CAT) assay. The cryptic promoter, further localized within a 153-bp fragment (fr153BN), exerted its effect in Ltk- and H-ras-transformed NIH3T3 (3T3-Hras) but not in parental NIH3T3 cells. The results suggested that the cryptic promoter was associated with a novel ras-responsive positive regulatory element, since fr153BN did not contain an AP-1-binding sequence motif, known as the ras-responsive enhancer element. The cryptic promoter activity of fr153BN was suppressed by an upstream 121-bp fragment (fr121SB) which contained a consensus sequence motif for binding of a repressor protein, GC factor, and regions showing sequence similarity with putative cis-acting repressor elements found in the vimentin gene. The degree of the suppression was greater in 3T3-Hras than in Ltk- cells. These positive and negative regulatory elements may be differentially involved in the regulation of ME491 antigen expression depending on the cell type.

Changes in the DNA polymerase beta gene expression during development of lung, brain, and testis suggest an involvement of the enzyme in DNA recombination

Changes in the expression pattern of DNA polymerase beta gene during rat lung, brain, and testis development have been investigated. A decrease in the level of beta-pol mRNA was observed during postnatal development of lung and brain. By contrast, an almost 20-fold increase in the level of beta-pol mRNA was observed during spermatogenesis. For most adult rat tissues the abundance of beta-pol mRNA was low compared with that of beta-actin mRNA. Northern blot analysis revealed four distinct transcripts hybridizing to beta-pol probes. At least two of them, 1.4 kb and 4.0 kb, were products of a beta-polymerase gene. The changes in the expression pattern during lung and brain development, and during spermatogenesis, suggest involvement of DNA polymerase beta in gap-filling DNA synthesis during recombination.

Characterization of the transcription-initiation site and of the promoter region within the 5' flanking region of the human aldolase C gene

Several aldolase C clones from a human genomic library have been identified using a mouse aldolase C cDNA as a hybridization probe. The most complete fragment of the clones identified is 14 kb long and contains the complete aldolase C gene. The nucleotide sequence analysis of more than 5 kb includes the intron/exon organization structure of the gene and the 3' and 5' flanking regions. Although no human cDNA is yet available, a canonical polyadenylation signal at the 3' end of the gene indicates the proximity of the poly(A) addition site. We have analyzed the 5' noncoding region by S1 mapping and primer-extension experiments. The transcription-initiation sites for the human aldolase C gene in brain tissue was located about 1300 bp upstream from the methionine initiation codon. Preliminary functional assays of the promoter by transfection into rat glioma cells have indicated that promoter elements lie between positions -161 and -416 from the start point of transcription.

Sequences downstream of the transcription initiation site modulate the activity of the murine dihydrofolate reductase promoter

The murine dihydrofolate reductase gene is regulated by a bidirectional promoter that lacks a TATA box. To identify the DNA sequences required for dihydrofolate reductase transcription, the activities of various templates were determined by in vitro transcription analysis. Our data indicate that sequences both upstream and downstream of the transcription initiation site modulate the activity of the dihydrofolate reductase promoter. We have focused on two regions downstream of the transcription initiation site that are important in determining the overall efficiency of the promoter. Region 1, which included exon 1 and part of intron 1, could stimulate transcription when placed in either orientation in the normal downstream position and when inserted upstream of the transcription start site. This region could also stimulate transcription in trans when the enhancer was physically separate from the promoter. Deletion of region 2, spanning 46 nucleotides of the 5' untranslated region, reduced transcriptional activity by fivefold. DNase I footprinting reactions identified protein-binding sites in both downstream stimulatory regions. Protein bound to two sites in region 1, both of which contain an inverted CCAAT box. The protein-binding site in the 5' untranslated region has extensive homology to binding sites in promoters that both lack (simian virus 40 late) and contain (adenovirus type 2 major late promoter and c-myc) TATA boxes.

A GC-box motif upstream of the B19 parvovirus unique promoter is important for in vitro transcription

Nucleotides upstream of the B19 parvovirus P6 promoter affect in vitro transcription in HeLa cell nuclear extracts. Comparison of the relative transcriptional strengths of equimolar mixes of plasmids containing the intact upstream sequence and plasmids containing deletions within these nucleotides identified several regions that affect transcription in vitro. A fragment containing two of five GC-box motifs which correspond to high-affinity SP1-binding sites was shown, by using a gel shift assay, to bind a HeLa cell factor (or factors). DNase I, methylation interference, and methylation protection footprinting demonstrated that the HeLa cell factor(s) bound to one of the two GC-box motifs within this fragment. Mutation of this GC box abolished factor binding and significantly reduces in vitro transcription from the P6 promoter. These results suggest that the B19 parvovirus promoter includes a complex regulatory region containing multiple sequences which affect promoter strength and that the GC-box motif is a major controlling sequence for in vitro transcription.

Fine structure analysis of the Chinese hamster AS gene encoding asparagine synthetase

Overlapping cDNAs for Chinese hamster ovary (CHO) asparagine synthetase (AS) were isolated from a library prepared from an AS-overproducing cell line. The sequence was determined and shown to contain an open reading frame encoding a protein of Mr 64,300. The predicted amino acid sequence for the CHO AS enzyme was compared to that of the human AS enzyme and found to be 95% homologous. A potential glutamine amide transfer domain, with sequence similarity to amidotransferases from bacteria and yeast, was identified in the N-terminal portion of the protein. The cDNAs were used to screen a library of phage containing wild type CHO DNA and the genomic AS sequences were detected on three overlapping phages. Determination of the fine structural organization showed that the CHO AS gene spanned 19 kilobases and was composed of 12 exons, three of which contained the glutamine amidotransferase domain. The 5' flanking sequences were highly G + C-rich and, like other housekeeping genes, lacked TATA and CAAT boxes.

Levels and size complexity of DNA polymerase beta mRNA in rat regenerating liver and other organs

A cDNA probe encoding DNA polymerase beta (beta-pol) was used to study the level and size complexity of beta-pol mRNA in regenerating rat liver and other rat tissues. An almost 2-fold increase in beta-pol mRNA was observed 18-24 h after partial hepatectomy. In most adult rat tissues (liver, heart, kidney, stomach, spleen, thymus, lung and brain) the abundance of beta-pol mRNA was low. In contrast, young brain and testes exhibited beta-pol mRNA levels 5- and 15-times higher, respectively. The observed changes in the level of beta-pol mRNA in regenerating rat liver and in developing brain are correlated with reported changes in DNA polymerase beta activity. Four different (4.0, 2.5, 2.2, 1.4 kb) transcripts hybridizing to beta-pol probe were found in all tissues examined. The 4.0 kb transcript was dominant for young and adult brain, whereas the 1.4 kb transcript was dominant for testes. The significance of these transcripts is discussed.

Molecular structure of the human asparagine synthetase gene

The human gene for asparagine synthetase has been isolated and the molecular organization has been determined by mapping and DNA sequencing of intron-exon boundaries. The gene spans 35 kb and contains 13 exons. The structure of the human gene has a high degree of similarity to that of the hamster asparagine synthetase gene, with identical positions for all but one of the intron regions. The 5' upstream region of this gene, like other housekeeping genes, lacks conventional TATA and CAAT boxes. Comparisons of the 5' upstream sequences of the human and hamster genes show limited similarity; however, both have a very high G + C content which may play a role in expression through DNA methylation.

Genomic organization of a sex specific gene: the primary sperm receptor of the mouse zona pellucida

The zona pellucida is an extracellular matrix which surrounds mammalian oocytes and plays an important role in the species-specific fertilization of mammals. The mouse zona is composed of three sulfated glycoproteins one of which, ZP3, serves as the primary sperm receptor in the initial interaction between egg and sperm. We have previously cloned a ZP3 cDNA and shown that the gene for ZP3 is expressed in a precise temporal and spatial fashion only in growing oocytes. We now describe the genomic organization of this gene whose expression is germ-line and female sex-specific. The gene contains 8 exons spanning approximately 8.6 kilobases in the mouse genome where it appears to be present as a single copy gene. S1 analyses of the 5' and 3' ends of the gene define the transcription start and stop sites and show that ZP3 mRNA has very short untranslated regions. Sequence determination has revealed a novel tandem repeat which is reiterated six times in the 5' flanking region and five times in the seventh intron.

Characterisation of cis-acting DNA sequences required for the expression of the chicken 5-aminolevulinate synthase gene in Xenopus oocytes

In this paper, we have constructed deletion and deletion-insertion mutations of the chicken 5-aminolevulinate synthase 5' flanking region and examined the expression of these constructs in microinjected Xenopus oocytes. Utilising this assay, we have delimited the boundary of the 5' flanking region required for expression to be 80 bp upstream from the transcription initiation site. A second major focus of this study has been to define the role of known putative cis-acting sequences in regulating 5-aminolevulinate synthase gene expression. Expression of the insertion-deletion mutants demonstrated that only a TATA box at position -28, and a single GC box at position -78 was necessary for expression of the 5-aminolevulinate synthase gene in Xenopus oocytes. This result is unusual in view of the current state of knowledge of the function of cis-acting sequence elements in transcriptional regulation.

Stage-specific selection of alternative transcriptional initiation sites from the 5C actin gene of Drosophila melanogaster

The transcription unit of the 5C actin gene exhibits a complex organization that is unique among the six actin genes of Drosophila melanogaster. Three different mRNA size classes showing distinct patterns of accumulation throughout development are detected on Northern blots. We have determined the structure of the various 5C actin transcripts by exon mapping using strand-specific RNA probes, primer extension analysis, and DNA sequences analysis of both cDNA and genomic clones. All the transcripts share a single protein-coding nucleotide sequence but are heterogeneous in the 5' and 3' untranslated regions. The 5' untranslated region of each transcript consists of either one of two small exons (exon 1 and exon 2) which are alternatively spliced to a single acceptor site 8 bp upstream from the translation initiation codon in exon 3. Results from primer extension analysis suggest that transcription can initiate from either exon 1 or exon 2, and also from a third site within exon 2. We detect an increase in the relative abundance of exon 1-containing transcripts at larval and pupal stages, as well as a change in the proportion of transcripts that initiate at either of the two exon 2 sites. Five polyadenylation sites have been found within three termination/processing regions that define the three size classes of polyadenylated transcripts. The results of our experiments indicate the existence in vivo of all possible combinations of 5' exon with 3' polyadenylation site. However, particular combinations of 5' initiation site and 3' polyadenylation site are preferred at certain developmental stages.

Structural organization of the mouse mitochondrial aspartate aminotransferase gene

Structural organization of the entire mouse mitochondrial aspartate aminotransferase (EC 2.6.1.1) gene was determined by analyzing the overlapping genomic clones obtained from a Charon 4A DNA library. The gene is 25 X 10(3) base-pairs long and contains ten exons interrupted by nine introns of various sizes. The 5' and 3'-flanking regions, the exact sizes and boundaries of the exon blocks including the transcription-initiation sites were determined. The 5' end of the gene lacks the prototypical 5' transcriptional regulatory sequence elements, such as TATA and CAAT boxes, but contains G + C-rich sequences, two putative binding sites for a cellular transcription factor, Sp1, and multiple transcription-initiation sites. Moreover, the sequences around the transcription-initiation sites are compatible with the formation of a number of potentially stable stem-loop structures. The leader sequence, which is essential for the transport of the protein into the mitochondria, is coded by the first exon and is separated from the mature protein by the first intron. The pyridoxal 5'-phosphate-binding domain, consisting of seven alternating beta-sheets and alpha-helical polypeptide strands, is separated by four introns present at the ends of alpha-helices. These genomic DNA structures suggest that the introns were not inserted into a previously uninterrupted coding sequence, but rather are products of evolution of the ancestral gene. However, a further correlation between the positions of introns relative to the well-defined structural domains of the mature protein was not obvious.

A new leader exon identified in the rat insulin-like growth factor II gene

In the rat insulin-like growth factor II gene, we identified an alternative 5'-untranslated sequence, in addition to the two sequences documented recently (1986) J. Mol. Biol. 192. 737-752, (1986) J. Biol. Chem. 261. 17138-17149. This sequence was encoded in an extreme 5' separate exon (E1 exon), the promoter region of which could be distinguished from others by features including lack of the TATA-box and GC-repeat, and heterogeneity in the transcriptional initiation.

CpG islands in vertebrate genomes

Although vertebrate DNA is generally depleted in the dinucleotide CpG, it has recently been shown that some vertebrate genes contain CpG islands, regions of DNA with a high G+C content and a high frequency of CpG dinucleotides relative to the bulk genome. In this study, a large number of sequences of vertebrate genes were screened for the presence of CpG islands. Each CpG island was then analysed in terms of length, nucleotide composition, frequency of CpG dinucleotides, and location relative to the transcription unit of the associated gene. CpG islands were associated with the 5' ends of all housekeeping genes and many tissue-specific genes, and with the 3' ends of some tissue-specific genes. A few genes contained both 5' and 3' CpG islands, separated by several thousand base-pairs of CpG-depleted DNA. The 5' CpG islands extended through 5'-flanking DNA, exons and introns, whereas most of the 3' CpG islands appeared to be associated with exons. CpG islands were generally found in the same position relative to the transcription unit of equivalent genes in different species, with some notable exceptions. The locations of G/C boxes, composed of the sequence GGGCGG or its reverse complement CCGCCC, were investigated relative to the location of CpG islands. G/C boxes were found to be rare in CpG-depleted DNA and plentiful in CpG islands, where they occurred in 3' CpG islands, as well as in 5' CpG islands associated with tissue-specific and housekeeping genes. G/C boxes were located both upstream and downstream from the transcription start site of genes with 5' CpG islands. Thus, G/C boxes appeared to be a feature of CpG islands in general, rather than a feature of the promoter region of housekeeping genes. Two theories for the maintenance of a high frequency of CpG dinucleotides in CpG islands were tested: that CpG islands in methylated genomes are maintained, despite a tendency for 5mCpG to mutate by deamination to TpG+CpA, by the structural stability of a high G+C content alone, and that CpG islands associated with exons result from some selective importance of the arginine codon CGX. Neither of these theories could account for the distribution of CpG dinucleotides in the sequences analysed. Possible functions of CpG islands in transcriptional and post-transcriptional regulation of gene expression were discussed, and were related to theories for the maintenance of CpG islands as "methylation-free zones" in germline DNA.

Nucleotide sequence of the promoter region of a tissue-specific human retroposon: comparison with its housekeeping progenitor

The intronless autosomal phosphoglycerate kinase gene (Pgk-2) is a functional retroposon expressed in a tissue-specific manner in the meiotic and postmeiotic stages of mammalian spermatogenesis. The nucleotide sequence of the promoter region of this gene and its transcription start point are compared with those of Pgk-1, an intron-containing, X-linked, housekeeping gene expressed constitutively in all somatic cells and premeiotic germ cells. The location of flanking direct repeats and apparent conservation of specific regulatory sequences suggest the Pgk-2 retroposon arose from reverse transcriptase-mediated processing of an aberrant Pgk-1 transcript that included the endogenous Pgk-1 promoter elements. Specific sequences that may be involved in mediating differences observed in both the level and cell-type specificity of expression of these genes in spermatogenesis are identified.

Nucleotide sequence and nuclease hypersensitivity of the Chinese hamster dihydrofolate reductase gene promoter region

We have sequenced the 1240 base pairs (bp) upstream from the translation start site of the hamster dihydrofolate reductase (DHFR) gene. The DNA in the 5' flanking region contains several elements that are homologous in both sequence and relative location to corresponding elements in the human and murine DHFR genes: an 11-bp element adjacent to the ATG codon, a 19-bp element that coincides with the major transcription start site, and two 29-bp upstream elements that are represented 4 times in the murine DHFR gene but only once in the human gene. Two clusters of short, G/C-rich elements conforming to the consensus binding sequence for the transcription factor Spl are located in the upstream region in all three genes. The symmetrical placement of the G/C boxes coincides with a symmetrical DNase I hypersensitive pattern in the chromatin, suggesting that the Spl protein may be involved in maintaining chromatin structure in this region.

alpha-Fetoprotein synthesis in transformed fetal rat liver cells

We have reported that transformed fetal liver cells produced a variant alpha-fetoprotein of 65K that differed from the mature alpha-fetoprotein of 69K and 73K in the polypeptide backbone. In the present study, we demonstrated that the biosynthetic pathway of the variant alpha-fetoprotein differed from that of the mature alpha-fetoprotein. The 65K variant was synthesized first as a preprotein of 49.5K which was processed to a polypeptide of 59K in the presence of microsomal membranes. The latter was the precursor of the variant alpha-fetoprotein found in cells and medium of transformed fetal liver cells. The 65K alpha-fetoprotein was encoded by a mRNA of 16S while mature AFP was encoded by a mRNA of 20S.