Role of the conserved AAUAAA sequence: four AAUAAA point mutants prevent messenger RNA 3' end formation

Cloning and expression of a hepatic microsomal glucose transport protein. Comparison with liver plasma-membrane glucose-transport protein GLUT 2

Antibodies raised against a 52 kDa rat liver microsomal glucose-transport protein were used to screen a rat liver cDNA library. Six positive clones were isolated. Two clones were found to be identical with the liver plasma-membrane glucose-transport protein termed GLUT 2. The sequence of the four remaining clones indicates that they encode a unique microsomal facilitative glucose-transport protein which we have termed GLUT 7. Sequence analysis revealed that the largest GLUT 7 clone was 2161 bp in length and encodes a protein of 528 amino acids. The deduced amino acid sequence of GLUT 7 shows 68% identity with the deduced amino acid sequence of rat liver GLUT 2. The GLUT 7 sequence is six amino acids longer than rat liver GLUT 2, and the extra six amino acids at the C-terminal end contain a consensus motif for retention of membrane-spanning proteins in the endoplasmic reticulum. When the largest GLUT 7 clone was transfected into COS 7 cells the expressed protein was found in the endoplasmic reticulum and nuclear membrane, but not in the plasma membrane. Microsomes isolated from the transfected COS 7 cells demonstrated an increase in their microsomal glucose-transport capacity, demonstrating that the GLUT 7 clone encodes a functional endoplasmic-reticulum glucose-transport protein.

Conditional defect in mRNA 3' end processing caused by a mutation in the gene for poly(A) polymerase

Maturation of most eukaryotic mRNA 3' ends requires endonucleolytic cleavage and polyadenylation of precursor mRNAs. To further understand the mechanism and function of mRNA 3' end processing, we identified a temperature-sensitive mutant of Saccharomyces cerevisiae defective for polyadenylation. Genetic analysis showed that the polyadenylation defect and the temperature sensitivity for growth result from a single mutation. Biochemical analysis of extracts from this mutant shows that the polyadenylation defect occurs at a step following normal site-specific cleavage of a pre-mRNA at its polyadenylation site. Molecular cloning and characterization of the wild-type allele of the mutated gene revealed that it (PAP1) encodes a previously characterized poly(A) polymerase with unknown RNA substrate specificity. Analysis of mRNA levels and structure in vivo indicate that shift of growing, mutant cells to the nonpermissive temperature results in the production of poly(A)-deficient mRNAs which appear to end at their normal cleavage sites. Interestingly, measurement of the rate of protein synthesis after the temperature shift shows that translation continues long after the apparent loss of polyadenylated mRNA. Our characterization of the pap1-1 defect implicates this gene as essential for mRNA 3' end formation in S. cerevisiae.

Expression of murine cyclin B1 mRNAs and genetic mapping of related genomic sequences

Two cDNAs that encode a protein with 87% identity to human cyclin B1 and that differ only in the length of their 3'-untranslated regions have been isolated from a 70Z/3B murine pre-B leukemia cell library. Three sizes of RNA transcripts were detected in Northern hybridization analyses of a variety of normal tissues and transformed cell lines using the cDNA inserts as probes. The expression of these RNAs can be modulated in tissue culture cell lines by physiologically relevant stimuli, increasing when cells are stimulated to proliferate and decreasing when cells are induced to differentiate. Moreover, RNAs from tissues that contain few proliferating cells have no detectable hybridizing transcripts. The coordinate regulation of these RNAs with other genes that are activated during the cell division cycle and the profound similarity of the predicted amino acid sequence to those of published cyclin B homologues indicate that these genes encode a murine cyclin B1. In Southern hybridization analysis of BALB/cAnPt genomic DNA digested with EcoRI, 12 fragments hybridized with the cDNA probes. Through Southern blot analyses of DNA from backcross and cogenic mice, recombinant inbred strains, and somatic cell hybrids, the genetic loci that produce the cyclin B1-related sequences (designated loci Cycb1-rs1 to Cycb1-rs9) were mapped on mouse chromosomes 5, 1, 17, 4, 14, 13, 7, X, and 8, respectively. Cycb1-rs6 (on chromosome 13) is discussed as the most likely candidate for an expressed structural gene locus.

Nopp140 shuttles on tracks between nucleolus and cytoplasm

Nopp140 is a nucleolar phosphoprotein of 140 kd that we originally identified and purified as a nuclear localization signal (NLS)-binding protein. Molecular characterization revealed a 10-fold repeated motif of highly conserved acidic serine clusters that contain an abundance of phosphorylation consensus sites for casein kinase II (CK II). Indeed, Nopp140 is one of the most phosphorylated proteins in the cell, and NLS binding was dependent on phosphorylation. Nopp140 was shown to shuttle between the nucleolus and the cytoplasm. Shuttling is likely to proceed on tracks that were revealed by immunoelectron microscopy. These tracks extend from the dense fibrillar component of the nucleolus across the nucleoplasm to some nuclear pore complexes. We suggest that Nopp140 functions as a chaperone for import into and/or export from the nucleolus.

Two distant upstream regions containing cis-acting signals regulating splicing facilitate 3'-end processing of avian sarcoma virus RNA

Retroviruses, pararetroviruses, and related retrotransposons generate terminally redundant RNAs by transcription of a template flanked by long terminal repeats in which initiation occurs within the 5' long terminal repeat sequences and 3'-end processing occurs within the 3' long terminal repeat sequences. Processing of avian sarcoma virus RNA is relatively inefficient; approximately 15% of the viral RNA transcripts are read-through products; i.e., they are not processed at the viral poly(A) addition site but at sites in the cellular sequence further downstream. In this report, we show that the efficiency of processing at the viral site is further reduced by deletion of two distant upstream sequences: (i) a 606-nucleotide sequence in the gag gene containing a cis-acting negative regulator of splicing and (ii) a 136-nucleotide sequence spanning the env 3' splice site. The deletion of either or both upstream regions increases the levels of read-through products of both unspliced and spliced viral RNA. In contrast, deletion of the src 3' splice site does not affect the efficiency of processing at the viral poly(A) addition site. The effects on 3'-end processing are not correlated either with distance from the promoter to the poly(A) addition site or with the overall level of viral RNA splicing. Substitution of the avian sarcoma virus poly(A) signal with the simian virus 40 early or late poly(A) signal relieves the requirement for the distant upstream sequences. We propose that cellular factors, which may correspond to splicing factors, bound to the upstream viral sequences may interact with factors bound at the avian sarcoma virus poly(A) signal to stabilize the polyadenylation-cleavage complex and allow for more efficient 3'-end processing.

Conditional defect in mRNA 3' end processing caused by a mutation in the gene for poly(A) polymerase

Maturation of most eukaryotic mRNA 3' ends requires endonucleolytic cleavage and polyadenylation of precursor mRNAs. To further understand the mechanism and function of mRNA 3' end processing, we identified a temperature-sensitive mutant of Saccharomyces cerevisiae defective for polyadenylation. Genetic analysis showed that the polyadenylation defect and the temperature sensitivity for growth result from a single mutation. Biochemical analysis of extracts from this mutant shows that the polyadenylation defect occurs at a step following normal site-specific cleavage of a pre-mRNA at its polyadenylation site. Molecular cloning and characterization of the wild-type allele of the mutated gene revealed that it (PAP1) encodes a previously characterized poly(A) polymerase with unknown RNA substrate specificity. Analysis of mRNA levels and structure in vivo indicate that shift of growing, mutant cells to the nonpermissive temperature results in the production of poly(A)-deficient mRNAs which appear to end at their normal cleavage sites. Interestingly, measurement of the rate of protein synthesis after the temperature shift shows that translation continues long after the apparent loss of polyadenylated mRNA. Our characterization of the pap1-1 defect implicates this gene as essential for mRNA 3' end formation in S. cerevisiae.

Different sequence elements are required for function of the cauliflower mosaic virus polyadenylation site in Saccharomyces cerevisiae compared with in plants

We show that the polyadenylation site derived from the plant cauliflower mosaic virus (CaMV) is specifically functional in the yeast Saccharomyces cerevisiae. The mRNA 3' endpoints were mapped at the same position in yeast cells as in plants, and the CaMV polyadenylation site was recognized in an orientation-dependent manner. Mutational analysis of the CaMV 3'-end-formation signal revealed that multiple elements are essential for proper activity in yeast cells, including two upstream elements that are situated more than 100 and 43 to 51 nucleotides upstream of the poly(A) addition site and the sequences at or near the poly(A) addition site. A comparison of the sequence elements that are essential for proper function of the CaMV signal in yeast cells and plants showed that both organisms require a distal and a proximal upstream element but that these sequence elements are not identical in yeast cells and plants. The key element for functioning of the CaMV signal in yeast cells is the sequence TAGTATGTA, which is similar to a sequence previously proposed to act in yeast cells as a bipartite signal, namely, TAG ... TATGTA. Deletion of this sequence in the CaMV polyadenylation signal abolished 3'-end formation in yeast cells, and a single point mutation in this motif reduced the activity of the CaMV signal to below 15%. These results indicate that the bipartite sequence element acts as a signal for 3'-end formation in yeast cells but only together with other cis-acting elements.

Molecular cloning of a cDNA coding for mouse liver xanthine dehydrogenase. Regulation of its transcript by interferons in vivo

The cDNA coding for xanthine dehydrogenase (XD) is isolated from mouse liver mRNA by cross-hybridization with a DNA fragment of the Drosophila melanogaster homologue. Two lambda bacteriophage overlapping clones represent the copy of a 4538-nucleotide-residue-long transcript with an open reading frame of 4005 nucleotide residues, coding for a putative polypeptide of 1335 amino acid residues. Comparison of the deduced amino acid sequence of the mouse XD with those of the Drosophila and the rat homologues shows a high conservation of this protein (55% identity between mouse and Drosophila, and 94% identity between mouse and rat). RNA blotting analysis demonstrates that interferon-alpha (IFN-alpha) and its inducers, i.e. poly(I).poly(C), bacterial lipopolysaccharide (LPS) and tilorone (2,7-bis-[2-(diethylamino)ethoxy]fluoren-9-one), increase the expression of XD mRNA in liver. Poly(I).poly(C) also induces XD mRNA in several other tissues in vivo. Protein synthesis de novo is not required for the elevation of XD mRNA after IFN-alpha treatment, since cycloheximide does not block the induction. The elevation of XD mRNA concentration is relatively fast and precedes the induction of both XD and xanthine oxidase (XO) enzymic activities.

Marek disease virus encodes a basic-leucine zipper gene resembling the fos/jun oncogenes that is highly expressed in lymphoblastoid tumors

Marek disease virus (MDV) is a herpesvirus of chickens that induces T lymphomas within 3 weeks of infection. The short latency and polyclonal nature of MDV-induced tumors have suggested that the virus may encode one or more direct-acting oncogenes. To date, however, no MDV-specific tumor antigens or candidate transforming genes have been demonstrated. In this paper, we report the identification of a MDV gene encoding a protein with homology to the leucine-zipper class of nuclear oncogenes. It also contains a proline-rich domain characteristic of another class of transcription factors. This gene, designated meq, maps to the long repeat of MDV and is one of the few genes that are highly expressed in MDV-induced T-cell tumors. To our knowledge, a herpesvirus gene closely related to the fos/jun family of oncogenes has not been reported previously.

Different sequence elements are required for function of the cauliflower mosaic virus polyadenylation site in Saccharomyces cerevisiae compared with in plants

We show that the polyadenylation site derived from the plant cauliflower mosaic virus (CaMV) is specifically functional in the yeast Saccharomyces cerevisiae. The mRNA 3' endpoints were mapped at the same position in yeast cells as in plants, and the CaMV polyadenylation site was recognized in an orientation-dependent manner. Mutational analysis of the CaMV 3'-end-formation signal revealed that multiple elements are essential for proper activity in yeast cells, including two upstream elements that are situated more than 100 and 43 to 51 nucleotides upstream of the poly(A) addition site and the sequences at or near the poly(A) addition site. A comparison of the sequence elements that are essential for proper function of the CaMV signal in yeast cells and plants showed that both organisms require a distal and a proximal upstream element but that these sequence elements are not identical in yeast cells and plants. The key element for functioning of the CaMV signal in yeast cells is the sequence TAGTATGTA, which is similar to a sequence previously proposed to act in yeast cells as a bipartite signal, namely, TAG ... TATGTA. Deletion of this sequence in the CaMV polyadenylation signal abolished 3'-end formation in yeast cells, and a single point mutation in this motif reduced the activity of the CaMV signal to below 15%. These results indicate that the bipartite sequence element acts as a signal for 3'-end formation in yeast cells but only together with other cis-acting elements.

Temporally correlated expression of nAChR genes during development of the mammalian retina

In situ hybridizations and RNase protection assays have been used to characterize nicotinic acetylcholine receptor (nAChR) gene expression in the developing and adult rat retina. At the earliest time examined (embryonic day 13) a low level of alpha-3 and beta-4 gene expression could be detected. During the next 48 hr there was a dramatic induction of the alpha-3, alpha-4, beta-2, beta-3 and beta-4 genes in the recently differentiated retinal ganglion cells. By post-natal day 4 we detected nAChR gene expression in the inner nuclear layer. In the adult retina, in situ hybridizations showed these genes are expressed by cells residing in the ganglion and inner nuclear layers. These results suggest a common regulatory mechanism for the induction of nAChR expression in retinal ganglion cells during development. In addition, the variety of nAChR genes expressed in the retina imply a relatively large number of different types of nAChRs can be expressed by these cells.

Drosophila melanogaster paramyosin : developmental pattern, mapping and properties deduced from its complete coding sequence

Several cDNA clones encoding the complete Drosophila paramyosin sequence, including two potential polyadenylation sites, have been obtained. Southern analysis and in situ hybridization to polytene chromosomes indicate that in Drosophila the paramyosin gene is single copy, located on the left arm of the third chromosome at region 66D14. Northern analyses show predominantly two different RNAs which are the products of the choice between the two alternative polyadenylation sites. The two species begin to be synthesized around 10 h of development when embryonic muscles are formed, expression peaking at the end of embryogenesis. The protein is first expressed at germ band shortening in association with muscle precursor cells. A second maximum of paramyosin RNA expression occurs at late pupal stages when the higher molecular weight form becomes more abundant. In young adults this species becomes the main transcript detected. The 102 kDa polypeptide sequence is highly similar to that of Caenorhabditis elegans paramyosin. The protein has a central alpha-helical coiled-coil rod, organized in 29 groups of four typical seven-residue repeats and flanked by two short non-alpha-helical regions. Several leucine zippers are located on the hydrophobic face of the alpha-helix in paramyosin which, together with disulfide bonds between cysteines, are probably involved in the stabilization of the dimer. The structural and functional properties of Drosophila paramyosin deduced from the sequence are compared with those of known invertebrate myosins and paramyosins.

Isolation, characterization, and expression of cDNAs encoding murine alpha-mannosidase II, a Golgi enzyme that controls conversion of high mannose to complex N-glycans

Golgi alpha-mannosidase II (GlcNAc transferase I-dependent alpha 1,3[alpha 1,6] mannosidase, EC 3.2.1.114) catalyzes the final hydrolytic step in the N-glycan maturation pathway acting as the committed step in the conversion of high mannose to complex type structures. We have isolated overlapping clones from a murine cDNA library encoding the full length alpha-mannosidase II open reading frame and most of the 5' and 3' untranslated region. The coding sequence predicts a type II transmembrane protein with a short cytoplasmic tail (five amino acids), a single transmembrane domain (21 amino acids), and a large COOH-terminal catalytic domain (1,124 amino acids). This domain organization which is shared with the Golgi glycosyl-transferases suggests that the common structural motifs may have a functional role in Golgi enzyme function or localization. Three sets of polyadenylated clones were isolated extending 3' beyond the open reading frame by as much as 2,543 bp. Northern blots suggest that these polyadenylated clones totaling 6.1 kb in length correspond to minor message species smaller than the full length message. The largest and predominant message on Northern blots (7.5 kb) presumably extends another approximately 1.4-kb downstream beyond the longest of the isolated clones. Transient expression of the alpha-mannosidase II cDNA in COS cells resulted in 8-12-fold overexpression of enzyme activity, and the appearance of cross-reactive material in a perinuclear membrane array consistent with a Golgi localization. A region within the catalytic domain of the alpha-mannosidase II open reading frame bears a strong similarity to a corresponding sequence in the rat liver endoplasmic reticulum alpha-mannosidase and the vacuolar alpha-mannosidase of Saccharomyces cerevisiae. Partial human alpha-mannosidase II cDNA clones were also isolated and the gene was localized to human chromosome 5.

Cloning of the mouse hepatitis virus (MHV) receptor: expression in human and hamster cell lines confers susceptibility to MHV

The cellular receptor for murine coronavirus mouse hepatitis virus (MHV)-A59 is a member of the carcinoembryonic antigen (CEA) family of glycoproteins in the immunoglobulin superfamily. We isolated a cDNA clone (MHVR1) encoding the MHV receptor. The sequence of this clone predicts a 424-amino-acid glycoprotein with four immunoglobulinlike domains, a transmembrane domain, and a short intracytoplasmic tail, MHVR1 is closely related to the murine CEA-related clone mmCGM1 (Mus musculus carcinoembryonic antigen gene family member). Western blot (immunoblot) analysis performed with antireceptor antibodies detected a glycoprotein of 120 kDa in BHK cells stably transfected with MHVR1. This corresponds to the size of the MHV receptor expressed in mouse intestine and liver. Human and hamster fibroblasts transfected with MHVR1 became susceptible to infection with MHV-A59. Like MHV-susceptible mouse fibroblasts, the MHVR1-transfected human and hamster cells were protected from MHV infection by pretreatment with monoclonal antireceptor antibody CC1. Thus, the 110- to 120-kDa CEA-related glycoprotein encoded by MHVR1 is a functional receptor for murine coronavirus MHV-A59.

Structure and expression of the human XPBC/ERCC-3 gene involved in DNA repair disorders xeroderma pigmentosum and Cockayne's syndrome

The human XPBC/ERCC-3 was cloned by virtue of its ability to correct the excision repair defect of UV-sensitive rodent mutants of complementation group 3. The gene appeared to be in addition implicated in the human, cancer prone repair disorder xeroderma pigmentosum group B, which is also associated with Cockayne's syndrome. Here we present the genomic architecture of the gene and its expression. The XPBC/ERCC-3 gene consists of at least 14 exons spread over approximately 45 kb. Notably, the donor splice site of the third exon contains a GC instead of the canonical GT dinucleotide. The promoter region, first exon and intron comprise a CpG island with several putative GC boxes. The promoter was confined to a region of 260 bp upstream of the presumed cap site and acts bidirectionally. Like the promoter of another excision repair gene, ERCC-1, it lacks classical promoter elements such as CAAT and TATA boxes, but it shares with ERCC-1 a hitherto unknown 12 nucleotide sequence element, preceding a polypyrimidine track. Despite the presence of (AU)-rich elements in the 3'-untranslated region, which are thought to be associated with short mRNA half-life actinomycin-D experiments indicate that the mRNA is very stable (t 1/2 greater than 3h). Southern blot analysis revealed the presence of XPBC/ERCC-3 cross-hybridizing fragments elsewhere in the genome, which may belong to a related gene.

A novel poly(A)-binding protein acts as a specificity factor in the second phase of messenger RNA polyadenylation

Polyadenylation of mRNA precursors by poly(A) polymerase depends on a specificity factor, CPF, recognizing the polyadenylation signal AAUAAA. This paper describes an apparently novel poly(A)-binding protein that acts as a second specificity factor, mediating the recognition of the growing poly(A) tail. A transition from a slow initiation phase of polyadenylation to rapid elongation occurs when the growing tail is long enough to serve as a binding site for the poly(A)-binding protein. Elongation of an RNA carrying a tail of 10 or more adenylate residues can occur independently of CPF. A sharp decrease in the poly(A) chain growth rate after the addition of approximately 200 adenylate residues invites speculations about a role of the poly(A)-binding protein in poly(A) tail length control.

The polypeptide encoded by the cDNA for human cell surface antigen Fas can mediate apoptosis

Mouse anti-Fas monoclonal antibody has a cytolytic activity on human cells that express the antigen. Complementary DNAs encoding the cell surface antigen Fas were isolated from a cDNA library of human T cell lymphoma KT-3 cells. The nucleotide sequence of the cDNAs revealed that the molecule coding for the Fas antigen determinant is a 319 amino acid polypeptide (Mr 36,000) with a single transmembrane domain. The extracellular domain is rich in cysteine residue, and shows a similarity to that of human tumor necrosis factor receptors, human nerve growth factor receptor, and human B cell antigen CD40. Murine WR19L cells or L929 cells transformed with the human Fas antigen cDNA were killed by the anti-Fas antibody in the process known as apoptosis.

The complex set of late transcripts from the Drosophila sex determination gene sex-lethal encodes multiple related polypeptides

Sex-lethal (Sxl), a key sex determination gene in Drosophila melanogaster, is known to express a set of three early transcripts arising during early embryogenesis and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs were distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). We have now analyzed the structures of the late Sxl transcripts by cDNA sequencing, Northern (RNA) blotting, primer extension, and RNase protection. The late transcripts appear to use a common 5' end but differ at their 3' ends by the use of alternative polyadenylation sites. Two of these sites lack canonical AATAAA sequences, and their use correlates in females with the presence of a functional germ line, suggesting possible tissue-specific polyadenylation. Besides the presence of the male-specific exon, no additional sex-specific splicing events were detected, although a number of non-sex-specific splicing variants were observed. In females, the various forms of late Sxl transcript potentially encode up to six slightly different polypeptides. All of the protein-coding differences occur outside the previously defined ribonucleoprotein motifs. One class of Sxl mRNAs also includes a second long ORF in the same frame as the first ORF but separated from it by a single ochre codon. The function of this second ORF is unknown. Significant amounts of apparently partially processed Sxl RNAs were observed, consistent with the hypothesis that the regulated Sxl splices occur relatively slowly.

Different classes of polyadenylation sites in the yeast Saccharomyces cerevisiae

This report provides an analysis of the function of polyadenylation sites from six different genes of the yeast Saccharomyces cerevisiae. These sites were tested for their ability to turn off read-through transcription into the URA3 gene in vivo when inserted into an ACT-URA3 fusion gene. The 3' ends of all polyadenylation sites inserted into the test system in their natural configuration are identical to the 3' ends of the chromosomal genes. We identified two classes of polyadenylation sites: (i) efficient sites (originating from the genes GCN4 and PHO5) that were functional in a strict orientation-dependent manner and (ii) bidirectional sites (derived from ARO4, TRP1, and TRP4) that had a distinctly reduced efficiency. The ADH1 polyadenylation site was efficient and bidirectional and was shown to be a combination of two polyadenylation sites of two convergently transcribed genes. Sequence comparison revealed that all efficient unidirectional polyadenylation sites contain the sequence TTTTTAT, whereas all bidirectional sites have the tripartite sequence TAG...TA (T)GT...TTT. Both sequence elements have previously been proposed to be involved in 3' end formation. Site-directed point mutagenesis of the TTTTTAT sequence had no effect, whereas mutations within the tripartite sequence caused a reduced efficiency for 3' end formation. The tripartite sequence alone, however, is not sufficient for 3' end formation, but it might be part of a signal sequence in the bidirectional class of yeast polyadenylation sites. Our findings support the assumption that there are at least two different mechanisms with different sequence elements directing 3' end formation in yeast.

Highly clustered zein gene sequences reveal evolutionary history of the multigene family

We have determined the nucleotide sequences of zein cDNA clones ZG14, ZG15, and ZG35. The three clones have 95 to 98% homology to the previously published sequence of clone A20, and 84% homology to sequences of the zein subfamily A30. Comparison of all sequences of the A30 and A20 subfamilies highlights the following features: the 5' nontranslated regions are 68 and 57 nucleotides in length for the A20- and A30-like mRNAs, respectively, and contain at least three repeats of the consensus sequence ACGAACAAta/gG; the majority of these genes are highly clustered as judged from pulsed-field gel electrophoresis of high molecular weight maize DNA. Furthermore, we discuss a model for the evolution of the multigene family which stresses the special importance of unequal crossingover and gene conversion in this system.

The complex set of late transcripts from the Drosophila sex determination gene sex-lethal encodes multiple related polypeptides

Sex-lethal (Sxl), a key sex determination gene in Drosophila melanogaster, is known to express a set of three early transcripts arising during early embryogenesis and a set of seven late transcripts occurring from midembryogenesis through adulthood. Among the late transcripts, male-specific mRNAs were distinguished from their female counterparts by the presence of an extra exon interrupting an otherwise long open reading frame (ORF). We have now analyzed the structures of the late Sxl transcripts by cDNA sequencing, Northern (RNA) blotting, primer extension, and RNase protection. The late transcripts appear to use a common 5' end but differ at their 3' ends by the use of alternative polyadenylation sites. Two of these sites lack canonical AATAAA sequences, and their use correlates in females with the presence of a functional germ line, suggesting possible tissue-specific polyadenylation. Besides the presence of the male-specific exon, no additional sex-specific splicing events were detected, although a number of non-sex-specific splicing variants were observed. In females, the various forms of late Sxl transcript potentially encode up to six slightly different polypeptides. All of the protein-coding differences occur outside the previously defined ribonucleoprotein motifs. One class of Sxl mRNAs also includes a second long ORF in the same frame as the first ORF but separated from it by a single ochre codon. The function of this second ORF is unknown. Significant amounts of apparently partially processed Sxl RNAs were observed, consistent with the hypothesis that the regulated Sxl splices occur relatively slowly.

Different classes of polyadenylation sites in the yeast Saccharomyces cerevisiae

This report provides an analysis of the function of polyadenylation sites from six different genes of the yeast Saccharomyces cerevisiae. These sites were tested for their ability to turn off read-through transcription into the URA3 gene in vivo when inserted into an ACT-URA3 fusion gene. The 3' ends of all polyadenylation sites inserted into the test system in their natural configuration are identical to the 3' ends of the chromosomal genes. We identified two classes of polyadenylation sites: (i) efficient sites (originating from the genes GCN4 and PHO5) that were functional in a strict orientation-dependent manner and (ii) bidirectional sites (derived from ARO4, TRP1, and TRP4) that had a distinctly reduced efficiency. The ADH1 polyadenylation site was efficient and bidirectional and was shown to be a combination of two polyadenylation sites of two convergently transcribed genes. Sequence comparison revealed that all efficient unidirectional polyadenylation sites contain the sequence TTTTTAT, whereas all bidirectional sites have the tripartite sequence TAG...TA (T)GT...TTT. Both sequence elements have previously been proposed to be involved in 3' end formation. Site-directed point mutagenesis of the TTTTTAT sequence had no effect, whereas mutations within the tripartite sequence caused a reduced efficiency for 3' end formation. The tripartite sequence alone, however, is not sufficient for 3' end formation, but it might be part of a signal sequence in the bidirectional class of yeast polyadenylation sites. Our findings support the assumption that there are at least two different mechanisms with different sequence elements directing 3' end formation in yeast.

Cellular transcripts encoded at a locus which permits retrovirus expression in mouse embryonic cells

Three independent recombinant retroviruses have been activated on insertion into the F2 locus of mouse F9 embryonal carcinoma cells. Each provirus has integrated downstream from the cellular F2 promoter, which is active in transient transfection assays using a chloramphenicol acetyltransferase reporter enzyme. The F2 promoter drives expression of a series of related transcripts in F9 and 3T3 cells, and a single 450-nt transcript in mouse tissues. F2 homologous sequences have been detected in the genomes of all mammalian species tested, and the 450-nucleotide (nt) F2 transcript is expressed in rat and human cells. Three pairs of differently sized F2 cDNA clones have been isolated and analyzed. The largest clones possess two 199-nt 98.5% identical repeats, one of which is present in the smaller clones, as well as the major 450-nt transcript. Activated proviral integration sites map to introns of the largest F2 cDNA clone. While none of the F2 cDNA contains a long open reading frame or homology to databank sequences, evidence suggests that the F2 locus encodes a constitutive function required at high levels, or represents an expressed but nonfunctional, single-copy element, conserved among mammals.

Elements involved in an in vitro block to transcription elongation at the end of the L1 mRNA family of adenovirus 2

Using the 3' end of the L1 mRNA family of adenovirus 2 (Ad2) as a model system, we investigated transcription elongation following a poly(A) signal in a cell-free system. The results show that RNA polymerase II can halt transcription elongation at a T-rich stretch in the non-coding DNA strand 20 nucleotides downstream of the poly(A) signal. The block to transcription elongation is enhanced when Sarkosyl is included in the elongation reaction. Deletion studies narrowed the region which directs the elongation block at the T-rich stretch, to an upstream fragment of 53 nucleotides that is very dA-rich and also contains a functional poly(A) signal. The deletion studies and analysis by site-directed mutagenesis indicate that in the present system, RNA secondary structure, the stretch of T's and the poly(A) signal are not the dominant elements responsible for the elongation block. The block to transcription elongation at the T-rich stretch was also shown to be 5 times more effective in an uninfected extract than in an Ad2 infected extract, which is reminiscent of the in vivo situation and is consistent with the suggestion that a trans-acting factor is involved in modulating the elongation block at the T-rich stretch.

Point mutations upstream of the yeast ADH2 poly(A) site significantly reduce the efficiency of 3'-end formation

The sequences directing formation of mRNA 3' ends in Saccharomyces cerevisiae are not well defined. This is in contrast to the situation in higher eukaryotes in which the sequence AAUAAA is known to be crucial to proper 3'-end formation. The AAUAAA hexanucleotide is found upstream of the poly(A) site in some but not all yeast genes. One of these is the gene coding for alcohol dehydrogenase, ADH2. Deletion or a double point mutation of the AAUAAA has only a small effect on the efficiency of the reaction, and in contrast to the mammalian system, it is most likely not operating as a major processing signal in the yeast cell. However, we isolated point mutations which reveal that a region located approximately 80 nucleotides upstream of the poly(A) site plays a critical role in either transcription termination, polyadenylation, or both. These mutations represent the first point mutations in yeasts which significantly reduce the efficiency of 3'-end formation.

Cloning and characterization of human placental catechol-O-methyltransferase cDNA

Catechol-O-methyltransferase (COMT) cDNA clones were isolated from a human placental cDNA library using synthetic oligonucleotides as probes. All four positive clones isolated contained an open reading frame, which potentially coded for a 24.4-kD polypeptide, presumably corresponding to the cytoplasmic form of the COMT (S-COMT). In addition to the S-COMT sequences, two of the clones carried extensions in the 5' end, which potentially coded for a 50-amino-acid peptide extending the S-COMT reading frame. This sequence contained a stretch of signal sequence-like hydrophobic amino acids in its amino terminus. The deduced human COMT polypeptide had 80% similarity with the previously characterized rat COMT. Expression of one of the cDNA clones in human K-562 cells resulted in cell clones with 3- to 10-fold increased COMT activity. Cell-free translation of transcripts synthesized in vitro from one of the short cDNAs yielded a 26-kD product, similar in size to human S-COMT. Translation of transcripts from one of the long cDNAs gave 30-kD and 26-kD polypeptides, suggesting translation initiation from two different AUG initiation codons. The 30-kD protein, but not the 25-kD protein, associated with microsomal membranes in translation lysates. A potential polyadenylation signal AATTAA was detected in the 3' ends of two of the clones 265 nucleotides downstream from the COMT translation termination codon. RNA blotting on human placental RNA revealed a 1.5-kb-long COMT-specific transcript. DNA analysis suggested that human, as well as rat, canine and monkey cells have one gene for COMT.

Point mutations upstream of the yeast ADH2 poly(A) site significantly reduce the efficiency of 3'-end formation

The sequences directing formation of mRNA 3' ends in Saccharomyces cerevisiae are not well defined. This is in contrast to the situation in higher eukaryotes in which the sequence AAUAAA is known to be crucial to proper 3'-end formation. The AAUAAA hexanucleotide is found upstream of the poly(A) site in some but not all yeast genes. One of these is the gene coding for alcohol dehydrogenase, ADH2. Deletion or a double point mutation of the AAUAAA has only a small effect on the efficiency of the reaction, and in contrast to the mammalian system, it is most likely not operating as a major processing signal in the yeast cell. However, we isolated point mutations which reveal that a region located approximately 80 nucleotides upstream of the poly(A) site plays a critical role in either transcription termination, polyadenylation, or both. These mutations represent the first point mutations in yeasts which significantly reduce the efficiency of 3'-end formation.

Involvement of long terminal repeat U3 sequences overlapping the transcription control region in human immunodeficiency virus type 1 mRNA 3' end formation

In retroviral proviruses, the poly(A) site is present in both long terminal repeats (LTRs) but used only in the 3' position. One mechanism to account for this selective poly(A) site usage is that LTR U3 sequences, transcribed only from the 3' poly(A) site, are required in the RNA for efficient processing. To test this possibility, mutations were made in the human immunodeficiency virus type 1 (HIV-1) U3 region and the mutated LTRs were inserted into simple and complex transcription units. HIV-1 poly(A) site usage was then quantitated by S1 nuclease analysis following transfection of each construct into human 293 cells. The results showed that U3 sequences confined to the transcription control region were required for efficient usage of the HIV-1 poly(A) site, even when it was placed 1.5 kb from the promoter. Although the roles of U3 in processing and transcription activation were separable, optimal 3' end formation was partly dependent on HIV-1 enhancer and SP1 binding site sequences.

Yeast CBP1 mRNA 3' end formation is regulated during the induction of mitochondrial function

Alternative mRNA processing is one mechanism for generating two or more polypeptides from a single gene. While many mammalian genes contain multiple mRNA 3' cleavage and polyadenylation signals that change the coding sequence of the mature mRNA when used at different developmental stages or in different tissues, only one yeast gene has been identified with this capacity. The Saccharomyces cerevisiae nuclear gene CPB1 encodes a mitochondrial protein that is required for cytochrome b mRNA stability. This 66-kDa protein is encoded by a 2.2-kb mRNA transcribed from CPB1. Previously we showed that a second 1.2-kb transcript is initiated at the CBP1 promoter but has a 3' end near the middle of the coding sequence. Furthermore, it was shown that the ratio of the steady-state level of 2.2-kb CBP1 message to 1.2-kb message decreases 10-fold during the induction of mitochondrial function, while the combined levels of both messages remain constant. Having proposed that regulation of 3' end formation dictates the amount of each CBP1 transcript, we now show that a 146-bp fragment from the middle of CBP1 is sufficient to direct carbon source-regulated production of two transcripts when inserted into the yeast URA3 gene. This fragment contains seven polyadenylation sites for the wild-type 1.2-kb mRNA, as mapped by sequence analysis of CBP1 cDNA clones. Deletion mutations upstream of the polyadenylation sites abolished formation of the 1.2-kb transcript, whereas deletion of three of the sites only led to a reduction in abundance of the 1.2-kb mRNA. Our results indicate that regulation of the abundance of both CBP1 transcripts is controlled by elements in a short segment of the gene that directs 3' end formation of the 1.2-kb transcript, a unique case in yeast cells.

Structure and expression of a gene encoding a putative GTP-binding protein identified by provirus integration in a transgenic mouse strain

The Mov-10 mouse strain was derived by infection of preimplantation embryos with the Moloney murine leukemia virus and carries one copy of the provirus in its germ line. Here we show that the provirus has integrated into an evolutionarily conserved gene that can code for a protein of 110 kDa containing the three consensus elements characteristic for GTP-binding proteins. The Mov-10 locus was expressed in a variety of cell types, including embryonal carcinoma and embryonic stem cells. Transcription of the gene was down-regulated about 10-fold when F9 embryonal carcinoma cells are differentiated into parietal endodermlike cells and about 2-fold when they are differentiated into visceral endodermlike cells. High levels of Mov-10 transcripts were also found at different stages of embryonal development and in the testes and thymus of adult animals. Expression was cell cycle controlled, with steady-state RNA levels significantly higher in growth-arrested than in growth-stimulated cells. The results suggest that the Mov-10 locus has an important function in development and/or control of cell proliferation. The provirus was shown to have integrated into intron 1 of the gene without disrupting expression, indicating that integration into intronic sequences of a transcription unit does not necessarily affect transcription. This result together with previous results from the Mov-13 mouse strain suggested that proviruses exert their mutagenic effect only by integration in specific sites, such as cis-regulatory DNA elements.

Involvement of long terminal repeat U3 sequences overlapping the transcription control region in human immunodeficiency virus type 1 mRNA 3' end formation

In retroviral proviruses, the poly(A) site is present in both long terminal repeats (LTRs) but used only in the 3' position. One mechanism to account for this selective poly(A) site usage is that LTR U3 sequences, transcribed only from the 3' poly(A) site, are required in the RNA for efficient processing. To test this possibility, mutations were made in the human immunodeficiency virus type 1 (HIV-1) U3 region and the mutated LTRs were inserted into simple and complex transcription units. HIV-1 poly(A) site usage was then quantitated by S1 nuclease analysis following transfection of each construct into human 293 cells. The results showed that U3 sequences confined to the transcription control region were required for efficient usage of the HIV-1 poly(A) site, even when it was placed 1.5 kb from the promoter. Although the roles of U3 in processing and transcription activation were separable, optimal 3' end formation was partly dependent on HIV-1 enhancer and SP1 binding site sequences.

Organization and nucleotide sequences of the human tyrosinase gene and a truncated tyrosinase-related segment

We have isolated and sequenced the gene encoding human tyrosinase, the key enzyme in pigment biosynthesis. The human tyrosinase gene contains five exons and spans more than 50 kb of DNA on chromosome segment 11q14----q21. We have also isolated a second segment in the human genome that is closely related to tyrosinase. The tyrosinase-related segment, located on 11p11.2----cen, contains only exons 4 and 5 plus adjacent noncoding regions. This segment is present in all human ethnic groups analyzed, and the noncoding nucleotide sequences shared by the 11q tyrosinase gene and the 11p tyrosinase-related segment differ by only 2.6%. This suggests that this segment of the tyrosinase gene was duplicated approximately 24 million years ago.

Molecular cloning of Taenia taeniaeformis oncosphere antigen genes

Infection of mice with the cestode Taenia taeniaeformis exhibits several important features common to other cestode infections, including the ability to vaccinate with crude antigen mixtures. Partial purification of the protective oncosphere antigens has been reported with a cutout from deoxycholate (DOC) acrylamide gels; this cutout was called fraction II (FII), and comprises approximately 10% of total DOC-soluble oncosphere antigen. Western blots of DOC gels probed with anti-FII antisera revealed a series of 3-5 discrete bands within the FII region. Further fractionation of the FII antigens on DOC gels was impractical due to limitations in supply of oncospheres, so a cDNA library was constructed from 150 ng of oncosphere mRNA and screened with alpha-FII antisera. Two distinct clone families were identified, oncA and oncB. Antibodies affinity-purified on either of two representative members, oncA1 and oncB1, recognised all the FII bands. Individual FII bands excised from a DOC gel resolved into an overlapping series of molecules when re-run on SDS-PAGE, indicating that each FII band consisted of several polypeptides of differing molecular weight. Immunoprecipitates resolved on SDS-PAGE revealed that alpha-FII recognised 3 major oncosphere antigens, of 62, 34 and 25 kDa; antisera against oncB precipitated both the 34- and 25-kDa antigens, whereas alpha-oncA antisera precipitated the 62-kDa antigen. We conclude that oncA and oncB encode the major antigens in the FII complex. The 62-kDa antigen encoded by oncA1 was the only common antigen precipitated by anti-FII and two other antisera raised against different protective extracts, suggesting that it may be a protective component in all three. Southern blot results indicate that oncA and oncB are distinct genes present at low copy number in the genome. Evidence is also presented suggesting that some cestode mRNAs, including oncA, may use variant polyadenylation signals.

Complete amino acid sequence of the type III isozyme of rat hexokinase, deduced from the cloned cDNA

Clones containing cDNA coding for the Type III isozyme of rat hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) were isolated from a library prepared in lambda gt10 with rat liver mRNA. Three clones were characterized. Their composite sequence includes the entire coding region for Type III hexokinase, 3' untranslated sequence extending into the polyadenylated region, and 80 bp of 5' untranslated sequence. Extensive similarity in sequence of N- and C-terminal halves of the enzyme, previously seen with the Type I isozyme, is consistent with the view that these 100-kDa mammalian hexokinases are the evolutionary result of duplication and fusion of a gene coding for an ancestral hexokinase having a molecular weight of approximately 50 kDa. Extensive similarities are seen between sequences of the Type I and III isozymes, and those reported for mammalian glucokinase (also called Type IV hexokinase) and for the hexokinase and glucokinase of yeast. Residues thought to be involved in catalytic function are highly conserved in all of these enzymes. Based on a quantitative comparison of sequence similarities, it is concluded that the 50-kDa mammalian glucokinase is more closely related to the 100-kDa mammalian enzymes than it is to the 50-kDa enzymes from yeast. One interpretation of this might be that the mammalian glucokinase arose by resplitting of the gene coding for the 100-kDa mammalian hexokinases.

Yeast CBP1 mRNA 3' end formation is regulated during the induction of mitochondrial function

Alternative mRNA processing is one mechanism for generating two or more polypeptides from a single gene. While many mammalian genes contain multiple mRNA 3' cleavage and polyadenylation signals that change the coding sequence of the mature mRNA when used at different developmental stages or in different tissues, only one yeast gene has been identified with this capacity. The Saccharomyces cerevisiae nuclear gene CPB1 encodes a mitochondrial protein that is required for cytochrome b mRNA stability. This 66-kDa protein is encoded by a 2.2-kb mRNA transcribed from CPB1. Previously we showed that a second 1.2-kb transcript is initiated at the CBP1 promoter but has a 3' end near the middle of the coding sequence. Furthermore, it was shown that the ratio of the steady-state level of 2.2-kb CBP1 message to 1.2-kb message decreases 10-fold during the induction of mitochondrial function, while the combined levels of both messages remain constant. Having proposed that regulation of 3' end formation dictates the amount of each CBP1 transcript, we now show that a 146-bp fragment from the middle of CBP1 is sufficient to direct carbon source-regulated production of two transcripts when inserted into the yeast URA3 gene. This fragment contains seven polyadenylation sites for the wild-type 1.2-kb mRNA, as mapped by sequence analysis of CBP1 cDNA clones. Deletion mutations upstream of the polyadenylation sites abolished formation of the 1.2-kb transcript, whereas deletion of three of the sites only led to a reduction in abundance of the 1.2-kb mRNA. Our results indicate that regulation of the abundance of both CBP1 transcripts is controlled by elements in a short segment of the gene that directs 3' end formation of the 1.2-kb transcript, a unique case in yeast cells.

Occlusion of the HIV poly(A) site

To investigate the selective use of poly(A) sites in the 3' long terminal repeat (LTR) but not the 5' LTR of retroviruses, we have studied the poly(A) site of the human immunodeficiency virus (HIV-1). Using hybrid HIV/alpha-globin gene constructs, we demonstrate that the HIV poly(A) site is inactive or occluded when adjacent to an active promoter, either the homologous HIV promoter or the alpha-globin gene promoter. Furthermore, this occlusion of the HIV poly(A) site occurs over a considerable distance of up to at least 500 bp. In contrast, two nonretroviral poly(A) sites [alpha-globin and a synthetic poly(A) site] are active when close to a promoter. We also show that a short fragment of approximately 60 nucleotides containing the HIV poly(A) site is fully active when placed at the 3' end of the human alpha-globin gene or within the rabbit beta-globin gene. This result rules out the requirement of more distant upstream elements for the activity of the HIV poly(A) site, as has been suggested for other viral poly(A) sites. Finally, we show that the GT-rich downstream region of the HIV poly(A) site confers poly(A) site occlusion properties on a synthetic poly(A) site. This result focuses attention on this more variable part of a poly(A) site in retroviruses as a possible general signal for poly(A) site occlusion.

Structure and expression of a gene encoding a putative GTP-binding protein identified by provirus integration in a transgenic mouse strain

The Mov-10 mouse strain was derived by infection of preimplantation embryos with the Moloney murine leukemia virus and carries one copy of the provirus in its germ line. Here we show that the provirus has integrated into an evolutionarily conserved gene that can code for a protein of 110 kDa containing the three consensus elements characteristic for GTP-binding proteins. The Mov-10 locus was expressed in a variety of cell types, including embryonal carcinoma and embryonic stem cells. Transcription of the gene was down-regulated about 10-fold when F9 embryonal carcinoma cells are differentiated into parietal endodermlike cells and about 2-fold when they are differentiated into visceral endodermlike cells. High levels of Mov-10 transcripts were also found at different stages of embryonal development and in the testes and thymus of adult animals. Expression was cell cycle controlled, with steady-state RNA levels significantly higher in growth-arrested than in growth-stimulated cells. The results suggest that the Mov-10 locus has an important function in development and/or control of cell proliferation. The provirus was shown to have integrated into intron 1 of the gene without disrupting expression, indicating that integration into intronic sequences of a transcription unit does not necessarily affect transcription. This result together with previous results from the Mov-13 mouse strain suggested that proviruses exert their mutagenic effect only by integration in specific sites, such as cis-regulatory DNA elements.

Homologous and unique G protein alpha subunits in the nematode Caenorhabditis elegans

A cDNA corresponding to a known G protein alpha subunit, the alpha subunit of Go (Go alpha), was isolated and sequenced. The predicted amino acid sequence of C. elegans Go alpha is 80-87% identical to other Go alpha sequences. An mRNA that hybridizes to the C. elegans Go alpha cDNA can be detected on Northern blots. A C. elegans protein that crossreacts with antibovine Go alpha antibody can be detected on immunoblots. A cosmid clone containing the C. elegans Go alpha gene (goa-1) was isolated and mapped to chromosome I. The genomic fragments of three other C. elegans G protein alpha subunit genes (gpa-1, gpa-2, and gpa-3) have been isolated using the polymerase chain reaction. The corresponding cosmid clones were isolated and mapped to disperse locations on chromosome V. The sequences of two of the genes, gpa-1 and gpa-3, were determined. The predicted amino acid sequences of gpa-1 and gpa-3 are only 48% identical to each other. Therefore, they are likely to have distinct functions. In addition they are not homologous enough to G protein alpha subunits in other organisms to be classified. Thus C. elegans has G proteins that are identifiable homologues of mammalian G proteins as well as G proteins that appear to be unique to C. elegans. Study of identifiable G proteins in C. elegans may result in a further understanding of their function in other organisms, whereas study of the novel G proteins may provide an understanding of unique aspects of nematode physiology.

Potential role of poly(A) polymerase in the assembly of polyadenylation-specific RNP complexes

To elucidate the mechanism by which poly(A) polymerase functions in the 3'-end processing of pre-mRNAs, polyadenylation-specific RNP complexes were isolated by sedimentation in sucrose density gradients and the fractions were analyzed for the presence of the enzyme. At early stages of the reaction, the RNP complexes were resolved into distinct peaks which sedimented at approximately 18S and 25S. When reactions were carried out under conditions which support cleavage or polyadenylation, the pre-mRNA was specifically assembled into the larger 25S RNP complexes. Polyclonal antibodies raised against the enzyme purified from a rat hepatoma, which have been shown to inhibit cleavage and polyadenylation (Terns, M., and Jacob, S. T., Mol. Cell. Biol. 9:1435-1444, 1989) also prevented assembly of the 25S polyadenylation-specific RNP complexes. Furthermore, formation of these complexes required the presence of a chromatographic fraction containing poly(A) polymerase. UV cross-linking analysis indicated that the purified enzyme could be readily cross-linked to pre-mRNA but in an apparent sequence-independent manner. Reconstitution studies with the fractionated components showed that formation of the 25S RNP complex required the poly(A) polymerase fraction. Although the enzyme has not been directly localized to the specific complexes, the data presented in this report supports the role of poly(A) polymerase as an essential component of polyadenylation-specific complexes which functions both as a structural and enzymatic constituent.

Molecular structure of Tcp-10 genes from the t complex responder locus

Male transmission ratio distortion (TRD) is a property of mouse t haplotypes which requires the t complex responder locus (Tcr). Tcr has been localized to a 70-160 kb region in t haplotypes. A candidate gene for the responder, called Tcp-10bt, has been cloned and is one member of a highly related gene family called Tcp-10 (formerly T66). Molecular evidence suggests that unique alternative splicing of the Tcp-10bt gene may be responsible for the mutant responder activity. Here we present the intron/exon structure of a representative Tcp-10 gene, and the characterization of alternative polyadenylation sites. The Tcp-10 genes contain 12 exons which span approximately 21 kb of DNA. At least six different polyadenylation sites are used, and none have a perfect consensus signal. This appears to be a common feature associated with testes-expressed transcripts. Since the gene we have analyzed is absent from many t haplotypes without apparent consequence, and no corresponding cDNAs have been isolated, it was speculated to be a pseudogene. However, no major sequence differences were found within the coding sequence to conclude that Tcp-10pst is a pseudogene.

A dissection of the cauliflower mosaic virus polyadenylation signal

Mutagenesis analysis of the polyadenylation [poly(A)] signal from the cauliflower mosaic virus (CaMV), a plant pararetrovirus, revealed striking differences to known vertebrate poly(A) signals. Our results show that (1) the AATAAA sequence is necessary for efficient cleavage at the poly(A) site, although the requirement for an authentic AATAAA might be less stringent in plant than in vertebrate cells; (2) surprisingly and in contrast to the majority of vertebrate poly(A) signals, the sequences downstream of the CaMV poly(A) site do not influence processing efficiency drastically although they affect the precision of cleavage; and (3) deletion of sequences upstream of the CaMV AATAAA sequence decreased processing at the CaMV site dramatically, suggesting the presence of one or several positively acting upstream elements. An oligonucleotide consisting of CaMV upstream sequences could induce the recognition of a normally silent exogenous poly(A) signal when inserted upstream of its AATAAA motif.

EM analysis of Drosophila chorion genes: amplification, transcription termination and RNA splicing

We have used the electron microscope to examine ultrastructurally several events occurring during the biogenesis of two very abundant chorion (eggshell) mRNA molecules in the follicle cells of Drosophila melanogaster--namely, selective gene amplification, transcription initiation and termination, and RNA processing. We find that the highly transcribed s36 and s38 genes are positioned in the central region of large, multi-forked amplified DNA structures. Transcript morphology is consistent with the known presence of a small intron at the 5' end of each gene. Mature transcripts are associated with spliceosomes, demonstrating that splice site selection occurs co-transcriptionally but that splicing is completed after transcript release from the template. We have also mapped the termination sites for the genes. The two genes exhibit efficient termination very near their poly(A) sites--within a 210 bp region for s36 and a 360 bp region for s38.