cis- and trans-acting sequences required for expression of simian virus 40 genes in mouse oocytes

Prospects Of Non-Coding Elements In Genomic Dna Based Gene Therapy

Gene therapy has made significant development since the commencement of the first clinical trials a few decades ago and has remained a dynamic area of research regardless of obstacles such as immune response and insertional mutagenesis. Progression in various technologies like next-generation sequencing (NGS) and nanotechnology has established the importance of non-coding segments of a genome, thereby taking gene therapy to the next level. In this review, we have summarized the importance of non-coding elements, highlighting the advantages of using full-length genomic DNA loci (gDNA) compared to complementary DNA (cDNA) or minigene, currently used in gene therapy. The focus of this review is to provide an overview of the advances and the future of potential use of gDNA loci in gene therapy, expanding the therapeutic repertoire in molecular medicine.

Shaping the nebulous enhancer in the era of high-throughput assays and genome editing

Since the 1st discovery of transcriptional enhancers in 1981, their textbook definition has remained largely unchanged in the past 37 years. With the emergence of high-throughput assays and genome editing, which are switching the paradigm from bottom-up discovery and testing of individual enhancers to top-down profiling of enhancer activities genome-wide, it has become increasingly evidenced that this classical definition has left substantial gray areas in different aspects. Here we survey a representative set of recent research articles and report the definitions of enhancers they have adopted. The results reveal that a wide spectrum of definitions is used usually without the definition stated explicitly, which could lead to difficulties in data interpretation and downstream analyses. Based on these findings, we discuss the practical implications and suggestions for future studies.

Secretion and assembly of zona pellucida glycoproteins by growing mouse oocytes microinjected with epitope-tagged cDNAs for mZP2 and mZP3

The zona pellucida (ZP) is a highly organized extracellular coat that surrounds all mammalian eggs. The mouse egg ZP is composed of three glycoproteins, called mZP1-3, that are synthesized, secreted, and assembled into a ZP exclusively by growing oocytes. Here, we microinjected epitope-tagged (Myc and Flag) cDNAs for mZP2 and mZP3 into the germinal vesicle (nucleus) of growing oocytes isolated from juvenile mice. Specific antibodies and laser scanning confocal microscopy were used to follow nascent, recombinant ZP glycoproteins in both permeabilized and nonpermeabilized oocytes. When such cDNAs were injected, epitope-tagged mZP2 (Myc-mZP2) and mZP3 (Flag-mZP3) were synthesized, packaged into large intracellular vesicles, and secreted by the vast majority of oocytes. Secreted glycoproteins were incorporated into only the innermost layer of the thickening ZP, and the amount of nascent glycoprotein in this region increased with increasing time of oocyte culture. Consistent with prior observations, the putative transmembrane domain at the C terminus of mZP2 and mZP3 was missing from nascent glycoprotein incorporated into the ZP. When the consensus furin cleavage site near the C terminus of mZP3 was mutated, such that it should not be cleaved by furin, secretion and assembly of mZP3 was reduced. On the other hand, mZP3 incorporated into the ZP lacked the transmembrane domain downstream of the mutated furin cleavage site, suggesting that some other protease(s) excised the domain. These results strongly suggest that nascent mZP2 and mZP3 are incorporated into only the innermost layer of the ZP and that excision of the C-terminal region of the glycoproteins is required for assembly into the oocyte ZP.

Lack of enhancer function in mammals is unique to oocytes and fertilized eggs

Previous studies have shown that the lack of novel coactivator activity in mouse oocytes and one-cell embryos (fertilized eggs) renders them incapable of utilizing Gal4:VP16-dependent enhancers (distal elements) but not promoters (proximal elements) in regulating transcription. This coactivator activity first appears in two- to four-cell embryos coincident with the major activation of zygotic gene expression. Here we show that whereas oocytes and fertilized eggs could utilize Sp1-dependent promoters, they could not utilize Sp1-dependent enhancers, although they showed promoter repression, which is a requirement for delineating enhancer function. In contrast, both Sp1-dependent promoters and enhancers were functional in two- to four-cell embryos. Furthermore, the same embryonic stem cell mRNA that provided the coactivator activity for Gal4:VP16-dependent enhancer function also provided Sp1-dependent enhancer function in oocytes. Therefore, the coactivator activity appears to be a requirement for general enhancer function. To determine whether the absence of enhancer function is a unique property of oocytes or a general property of other terminally differentiated cells, transcription was examined in terminally differentiated hNT neurons and their precursors, undifferentiated NT2 stem cells. The results showed that both cell types could utilize enhancers and promoters. Thus, in mammals, the lack of enhancer function appears to be unique to oocytes and fertilized eggs, suggesting that it provides a safeguard against premature activation of genes prior to zygotic gene expression during development.

A unique role for enhancers is revealed during early mouse development

Transcription and replication of genes in mammalian cells always requires a promoter or replication origin, respectively, but the ability of enhancers to stimulate these regulatory elements and the interactions that mediate this stimulation are developmentally acquired. The primary function of enhancers is to prevent repression, which appears to result from particular components of chromatin structure. Factors responsible for this repression are present in the maternal nucleus of oocytes and its descendant, the maternal pronucleus of mouse 1-cell embryos and in mouse 2-cell embryos, but are absent in the paternal pronucleus. Thus, enhancers are not needed to achieve efficient transcription and replication in paternal pronuclei. However, enhancers, even in the presence of their specific activation protein, are inactive prior to formation of a 2-cell embryo, suggesting that a coactivator essential for enhancer function is not available until zygotic gene expression begins. Furthermore, enhancer stimulation of transcription appears to be mediated through a promoter transcription factor, but this interaction can change as cells undergo differentiation, switching from a TATA-box independent to a TATA-box dependent mode.

Regulation of gene expression at the beginning of mammalian development

The maternal to zygotic transition can be viewed as a cascade of events that begins when fertilization triggers the zygotic clock that delays early ZGA until formation of a 2-cell embryo. Early ZGA, in turn, appears to be required for expression of late ZGA, and late ZGA is required to form a 4-cell embryo. ZGA in mammals is a time-dependent mechanism rather than a cell cycle-dependent mechanism that delays both transcription and translation of nascent transcripts. Thus, zygotic gene transcripts appear to be handled differently than maternal mRNA, a phenomenon also observed in Xenopus (55). The length of this delay is species-dependent, occurring at the 2-cell stage in mice, the 4-8-cell stage in cows and humans, and the 8-16-cell stage in sheep and rabbits (4). However, concurrent with formation of a 2-cell embryo in the mouse and rabbit (47,56), perhaps in all mammals, a general chromatin-mediated repression of promoter activity appears. Repression factors are inherited by the maternal pronucleus from the oocyte but are absent in the paternal pronucleus and not available until sometime during the transition from a late 1-cell to a 2-cell embryo. This means that paternally inherited genes are exposed to a different environment in fertilized eggs than are maternally inherited genes, a situation that could contribute to genomic imprinting. Chromatin-mediated repression of promoter activity prior to ZGA is similar to what is observed during Xenopus embryogenesis (31,32) and ensures that genes are not expressed until the appropriate time in development when positive acting factors, such as enhancers, can relieve this repression. The ability to use enhancers appears to depend on the acquisition of specific co-activators at the 2-cell stage in mice and perhaps later in other mammals (47,56), concurrent with ZGA. Even then, the mechanism by which enhancers communicate with promoters changes during development (Fig. 2), providing an opportunity for enhancer-mediated stimulating of TATA-less promoters (e.g. housekeeping genes) early during development while eliminating this mechanism later during development.(ABSTRACT TRUNCATED AT 400 WORDS)

TATA-dependent enhancer stimulation of promoter activity in mice is developmentally acquired

Herpes simplex virus (HSV) thymidine kinase (tk) promoter activity depends on four transcription factor binding sites, one of which is a TATA box sequence, and the presence of either a cis-acting enhancer sequence or a transactivator protein. Studies presented here show that this TATA box was required for promoter activity only after cells began to differentiate and then only when promoter activity was stimulated by either an enhancer or a transactivator. When the HSV tk promoter was utilized by mouse embryos from the one-cell to eight-cell stage of development or by undifferentiated mouse embryonic stem cells, disruption of the HSV tk TATA box by site-specific mutations did not reduce promoter activity. This was true even when HSV tk promoter activity was stimulated strongly by either the embryo-responsive polyomavirus F101 enhancer or its natural transactivator, the HSV ICP4 gene product. However, stimulated expression was dependent on a distal Sp1 DNA binding site. Similarly, disruption of the TATA box did not reduce tk promoter activity in primary mouse embryonic fibroblasts or in immortalized 3T3 mouse fibroblasts; in fact, promoter activity was increased up to 2.6-fold. However, in these differentiated cells, stimulation of the HSV tk promoter by either the F101 enhancer or ICP4 protein required the TATA box. HSV tk promoter activity also was dependent on its TATA box in the mouse oocyte, a terminally differentiated cell with an endogenous transactivating activity. These results reveal that the need for a TATA box is developmentally acquired and depends on at least two parameters: the differentiated state of the cell and stimulation of the promoter by either an enhancer or a transactivator.

TATA-dependent enhancer stimulation of promoter activity in mice is developmentally acquired

Herpes simplex virus (HSV) thymidine kinase (tk) promoter activity depends on four transcription factor binding sites, one of which is a TATA box sequence, and the presence of either a cis-acting enhancer sequence or a transactivator protein. Studies presented here show that this TATA box was required for promoter activity only after cells began to differentiate and then only when promoter activity was stimulated by either an enhancer or a transactivator. When the HSV tk promoter was utilized by mouse embryos from the one-cell to eight-cell stage of development or by undifferentiated mouse embryonic stem cells, disruption of the HSV tk TATA box by site-specific mutations did not reduce promoter activity. This was true even when HSV tk promoter activity was stimulated strongly by either the embryo-responsive polyomavirus F101 enhancer or its natural transactivator, the HSV ICP4 gene product. However, stimulated expression was dependent on a distal Sp1 DNA binding site. Similarly, disruption of the TATA box did not reduce tk promoter activity in primary mouse embryonic fibroblasts or in immortalized 3T3 mouse fibroblasts; in fact, promoter activity was increased up to 2.6-fold. However, in these differentiated cells, stimulation of the HSV tk promoter by either the F101 enhancer or ICP4 protein required the TATA box. HSV tk promoter activity also was dependent on its TATA box in the mouse oocyte, a terminally differentiated cell with an endogenous transactivating activity. These results reveal that the need for a TATA box is developmentally acquired and depends on at least two parameters: the differentiated state of the cell and stimulation of the promoter by either an enhancer or a transactivator.

Activity of a microinjected inducible murine hsp68 gene promoter depends on plasmid configuration and the presence of heat shock elements in mouse dictyate oocytes but not in two-cell embryos

After fertilization in the mouse, the zygotic genome is activated in two-cell embryos by the spontaneous expression, among other genes, of the major inducible heat shock gene, hsp68, in the absence of heat-inducibility of heat shock genes. To obtain information on this phenomenon, we have probed one- and two-cell embryo's ability to express microinjected reporter DNA constructs, containing the Escherichia coli lacZ gene driven by promoters from early SV40 genes, the human beta-actin gene, and the normal or HSE-deleted mouse hsp68 gene. Activity of these promoters was also tested in mouse granulosa cells and dictyate oocytes, as a function of circular/linear construct configuration and occurrence of heat shock. The hsp68 promoter was heat-inducible in both granulosa cells and oocytes. Its heat activation required the presence of HSEs and, in the oocytes, of construct linear configuration. In the embryos however, this promoter was expressed independently of the presence of HSEs and of construct configuration, and its activity was not affected by heat shock. When constructs with early SV40 and beta-actin promoters were injected into one-cell embryos, they appeared to be inactivated with the first embryonic cleavage, in agreement with previous observations [Wiekowski et al., 1992]. By contrast, both normal and HSE-deleted hsp68 promoters maintained their activity through the first cleavage, providing the first evidence of a gene escaping such transcriptional repression. Present results confirm previous findings on hsp68 expression during early mouse development, and suggest that this activation is mediated by a factor(s) other than HSF.

Genetic manipulation of mammalian dictyate oocytes: factors affecting transient expression of microinjected DNA templates

Transcription of exogenous DNA templates in mouse ovarian oocytes was investigated by microinjecting constructs encoding for the Escherichia coli lacZ gene under control of promoters from: 1) the mouse hsp68 gene; 2) the human beta-actin gene; and 3) simian virus 40 (SV40) early genes. Various amounts of circular or linear DNA constructs were injected into dictyate oocyte nuclei at different stages of follicle growth, and the beta-galactosidase activity was then cytochemically evaluated in single cells. In middle-sized growing oocytes, expression of circular constructs was observed with amounts of DNA ranging from 50 to 10(3) plasmid copies/nucleus and was first observed 10-12 hr after injection. Maximal expression levels were reached by 17 hr after injection and were specific for the constructs used. Circular constructs containing the hsp68 and early SV40 promoters were expressed at similar levels in small- and middle-sized growing oocytes, while the construct carrying the beta-actin promoter was expressed only in small-sized cells. In contrast to growing oocytes, these constructs were never expressed in fully grown oocytes. DNA linearization depressed construct activity regardless of the site of cleavage. These results show that: 1) lacZ is a valuable reporter gene in the analysis of eukaryotic promoter activity in dictyate mouse oocytes; 2) transient construct expression requires the injection of DNA in circular form; and 3) the expression efficiency of different DNA templates is dependent on the presence of a specific promoter and on the differentiation stage of oocytes analyzed.

The replication activation potential of selected RNA polymerase II promoter elements at the simian virus 40 origin

Binding sites for cellular transcription factors were placed near the simian virus 40 origin of replication, and their effect on replication and TATA-dependent transcription was measured in COS cells. The hierarchy of transcriptional stimulation changed when the plasmids replicated. Only one of seven inserted sequences, a moderately weak transcription element, stimulated replication detectably. However, when two nonstimulatory sites were present in multiple copies they did activate replication. Multiple sites for the chimeric activator GAL4-VP16 did not stimulate replication even though transcription was stimulated strongly. The results indicate that the ability of a binding site to stimulate replication from the simian virus 40 ori is not based on its transcriptional activation potential but is instead related to a separate replication activation potential that can be increased by having multiple sites.

The replication activation potential of selected RNA polymerase II promoter elements at the simian virus 40 origin

Binding sites for cellular transcription factors were placed near the simian virus 40 origin of replication, and their effect on replication and TATA-dependent transcription was measured in COS cells. The hierarchy of transcriptional stimulation changed when the plasmids replicated. Only one of seven inserted sequences, a moderately weak transcription element, stimulated replication detectably. However, when two nonstimulatory sites were present in multiple copies they did activate replication. Multiple sites for the chimeric activator GAL4-VP16 did not stimulate replication even though transcription was stimulated strongly. The results indicate that the ability of a binding site to stimulate replication from the simian virus 40 ori is not based on its transcriptional activation potential but is instead related to a separate replication activation potential that can be increased by having multiple sites.

Transcriptional selectivity in early mouse embryos: a qualitative study

The mouse zygotic genome is activated at the 2-cell stage. At this stage, microinjected DNA can be expressed and its transcription, analysed qualitatively with LacZ reporter genes, has the following characteristics (i) Sp1-sensitive promoters are active; (ii) the SV40 early promoter does not require upstream enhancers; (iii) genes driven by the -447, +563 region of murine leukemia virus (M-MuLV) are repressed and; (iv) activation of promoters is possible as shown for the promoter of acetylcholine receptor alpha-subunit by MyoD. This transactivation can occur before the formation of the zygotic genome. The transcriptional selectivity of 2-cell embryos also characterizes oocytes and 4-cell embryos. Therefore the elements involved are present in the oocytes and they persist after fertilization. This transcriptional selectivity has numerous common characteristics with that in EC cells, and may be indicative of a genetic control program specific for multipotential cells.

c-mos expression in mouse oocytes is controlled by initiator-related sequences immediately downstream of the transcription initiation site

We have employed transient expression assays to analyze the sequences that direct c-mos transcription in mouse oocytes. Plasmids containing the chloramphenicol acetyltransferase (CAT) gene fused to either a 2.4-kb or a 731-bp fragment from the 5'-flanking region of c-mos produced similar levels of CAT activity when injected into nuclei of growing oocytes. BAL 31 deletions revealed that sequences up to 20 bp upstream of the major transcription start site could be removed without any significant loss of CAT activity. Promoter activity only decreased when these deletions closely approached the transcription start site, which was mapped at 53 nucleotides upstream of the first ATG in the c-mos open reading frame. On the other hand, deletion of sequences within 20 nucleotides downstream of the transcription initiation site resulted in a 10-fold reduction in CAT expression. A similar decrease in promoter activity was observed as a result of point mutations in these 5' untranslated sequences. Thus, sequences immediately downstream of the transcription start site, including a consensus sequence (PyPyCAPyPyPyPyPy) present in the initiator elements of several genes, appear to regulate c-mos expression in mouse oocytes. Reverse transcription-polymerase chain reaction analysis of RNA from injected oocytes showed that this regulation is manifest at the transcriptional level. Expression of c-mos in mouse oocytes thus appears to be directed by a simple promoter consisting only of sequences immediately surrounding the transcription start site, including an initiator element in the untranslated leader.

c-mos expression in mouse oocytes is controlled by initiator-related sequences immediately downstream of the transcription initiation site

We have employed transient expression assays to analyze the sequences that direct c-mos transcription in mouse oocytes. Plasmids containing the chloramphenicol acetyltransferase (CAT) gene fused to either a 2.4-kb or a 731-bp fragment from the 5'-flanking region of c-mos produced similar levels of CAT activity when injected into nuclei of growing oocytes. BAL 31 deletions revealed that sequences up to 20 bp upstream of the major transcription start site could be removed without any significant loss of CAT activity. Promoter activity only decreased when these deletions closely approached the transcription start site, which was mapped at 53 nucleotides upstream of the first ATG in the c-mos open reading frame. On the other hand, deletion of sequences within 20 nucleotides downstream of the transcription initiation site resulted in a 10-fold reduction in CAT expression. A similar decrease in promoter activity was observed as a result of point mutations in these 5' untranslated sequences. Thus, sequences immediately downstream of the transcription start site, including a consensus sequence (PyPyCAPyPyPyPyPy) present in the initiator elements of several genes, appear to regulate c-mos expression in mouse oocytes. Reverse transcription-polymerase chain reaction analysis of RNA from injected oocytes showed that this regulation is manifest at the transcriptional level. Expression of c-mos in mouse oocytes thus appears to be directed by a simple promoter consisting only of sequences immediately surrounding the transcription start site, including an initiator element in the untranslated leader.

Regulation of gene expression in preimplantation mouse embryos: effects of the zygotic clock and the first mitosis on promoter and enhancer activities

Previous studies have reported that promoters requiring enhancers for full activity in mammalian somatic cells also require enhancers when injected into mouse two-cell embryos, whereas the same promoters can be expressed just as efficiently in the absence of an enhancer when injected into arrested one-cell embryos. Experiments were designed to determine whether this phenomenon reflected normal developmental changes at the beginning of mammalian development, or simply differences in the physiological states of these cells under the experimental conditions employed. The activity of three different promoters that function in a wide variety of mammalian cells was measured both in embryos whose morphological development was arrested and in embryos that continued development in vitro. Expression of the injected gene was related to the onset of zygotic gene expression ("zygotic clock"), the phase of the cell proliferation cycle, the use of aphidicolin to arrest cell proliferation, and formation of two-cell embryos in vitro and in vivo. The results demonstrated that promoter activity was tightly linked to zygotic gene expression, while the need for enhancers to stimulate promoter activity depended only on formation of a two-cell embryo. These results further support the hypothesis that the first mitosis induces a general repression of promoters prior to initiation of zygotic gene expression that is relieved specifically by enhancers.

Transcription factor LSF binds two variant bipartite sites within the SV40 late promoter

The HeLa transcription factor LSF has been purified by heparin-agarose and DNA affinity chromatography, and its DNA binding and transcription properties have been characterized. LSF is a 63-kD polypeptide that binds to two distinct bipartite sites within the SV40 promoter region. One binding site consists of GC motifs 2 and 3 within the 21-bp repeats (LSF-GC site), and the other consists of sequences centered 44 bp upstream of the major late initiation site, L325 (LSF-280 site). Four guanine residues within the LSF-GC site, when methylated, strongly interfere with LSF binding. Alteration of the spacing, but not the sequence, between the two directly repeated GC motifs dramatically reduces the binding affinity of LSF for the site. Thus, LSF appears to recognize directly repeated GC motifs, when their center-to-center distance is 10 bp. The LSF-GC and LSF-280 sites share limited sequence homology. Only half of the LSF-280 site contains a short GC-rich sequence homologous to the GC motif. However, the binding affinity of LSF to the two sites is similar. LSF activates transcription from the SV40 late promoter in vitro from initiation site L325, via its binding to the template DNA.

Sequence signals in eukaryotic upstream regions

Two DNA sequence elements are known to recur frequently upstream of eukaryotic polymerase II-transcribed genes. The TATAAA, at position -40, specifies the transcription initiation site. The GGCCAATCT is less frequent around -80. Sequence analysis of upstream regions reveals that the underlined yeast UAS2 consensus sequence, TGATTGGT, is also very frequent at -80 in higher polymerase II-transcribed animal sequences. The underlined CCAAT box and yeast UAS sequences are complementary. Structural analysis suggests some symmetry in their DNA structures. Upstream of the TATAAT-rich region there is an abundance of GC sequences. Analysis of nucleotide tracts indicates that these are preferentially flanked by their complementary nucleotides with a pyrimidine-purine junction, i.e., TTAN, CCGn, CnGG, TnAA. Here, I discuss DNA structural consideration in upstream regions along with protein readout of the major and minor groove information content. These sequence-structure aspects are put in the general context of protein (factors)-DNA (elements) recognition and regulation.

The need for enhancers in gene expression first appears during mouse development with formation of the zygotic nucleus

Microinjection of the firefly luciferase gene coupled to a thymidine kinase (tk) promoter provided a quantitative assay to evaluate the requirements for gene expression in individual mouse oocytes and embryos. Polyoma virus (PyV) enhancers had no effect on the level of gene expression or competition for transcription factors as long as the DNA remained either in the oocyte germinal vesicle or the pronuclei of one-cell embryos. Expression of injected genes could be observed in pronuclei because the signal that normally triggers zygotic gene expression in two-cell embryos still occurred in one-cell embryos arrested in S phase. However, when the tk promoter was injected into zygotic nuclei of two-cell embryos, enhancers increased the number of embryos that expressed luciferase as well as the level of luciferase activity per embryo. PyV enhancer mutation F101, selected for growth in mouse embryonal carcinoma F9 cells, stimulated expression in developing two-cell embryos about seven times better than the wild-type PyV enhancer and competed effectively for factors required for transcription. These results were consistent with the fact that enhancers are required to activate the PyV origin of DNA replication in developing two-cell embryos but not in one-cell embryos. The maximum levels of gene expression in oocytes, one-cell embryos, and developing two-cell embryos (1:67:21) were inversely related to the extent of chromatin assembly, but the need for enhancers was independent of chromatin assembly. Therefore, it appears that the need for enhancers to activate promoters or origins of replication results from some negative regulatory factor that first appears as a component of zygotic nuclear structure.

Transactivation of both early and late simian virus 40 promoters by large tumor antigen does not require nuclear localization of the protein

The early gene product of simian virus 40, large tumor antigen (T antigen), is required for the onset of viral replication. This protein has also been reported to transactivate viral late gene expression, independently of replication. In this study I have used a vector that permits simultaneously a precise quantitation of simian virus 40 early and late promoter activity with a single nuclease S1 mapping probe. The results show that T antigen can activate the early promoter as well as the late promoter and that only on replicating templates does a shift occur in the ratio of late-to-early transcription. This simultaneous transactivation of early and late promoters occurs in human (HeLa) and monkey (CV-1) cells but does not occur in mouse embryonal carcinoma cells. It is seen with either wild-type T antigen or with a T antigen protein that carries a mutation in the nuclear localization signal. The mutant protein cannot bring about an early-to-late shift, consistent with its inability to support viral replication.

The need for enhancers is acquired upon formation of a diploid nucleus during early mouse development

The activity of the polyoma virus (PyV) origin of DNA replication was used as a sensitive assay for enhancer function in one- and two-cell mouse embryos by injecting embryos with plasmid DNA containing different PyV ori configurations, allowing them to continue development in vitro, and then measuring plasmid DNA replication. Replication always required the PyV origin 'core' sequence in cis and PyV large tumor antigen (T-Ag) in trans. In developing two-cell embryos, DNA replication also required an enhancer in cis. Two copies of part of PyV enhancer 3 (beta element) was sevenfold better than one copy, and enhancer 3 was better than enhancer 1 + 2 (alpha element). Competition between ori configurations suggested that enhancers bound specific proteins required for replication and transcription. In contrast, DNA injected into one-cell embryos did not need an enhancer for replication, and no competition for replication factors was observed between different ori configurations. In fact, ori core replicated about ninefold better in one-cell embryos than the complete origin did in developing two-cell embryos. Therefore, core contains all the cis-acting information necessary to initiate DNA replication. Because one-cell embryos that replicated injected DNA retained their pronuclei and remained one-cell embryos, enhancers are not needed in mammalian development until a diploid nucleus is formed.

Formation of stable transcription complexes as assayed by analysis of individual templates

Conditions were established where transient transfection of two marker genes resulted in the expression of one or the other, but not both, in individual cells as assayed by immunofluorescence. Thus, the expression from a single cell reflects the activity of single active transcription templates. Under these conditions, a vector encoding the simian virus 40 large tumor antigen (SV40 T-Ag) driven by the SV40 enhancer and early promoter was transfected into CV-1, L, or HeLa cells yielding, for all three cell types, about 10-30% T-Ag-positive cells as assayed by immunofluorescence. Similar vectors containing either mutated or deleted SV40 enhancers also gave T-Ag-positive cells, but at about 1/100 the frequency. Quantitative analysis showed that T-Ag-positive cells produced about the same amount of T-Ag whether or not an active enhancer was present. Chloramphenicol acetyltransferase-encoding vectors gave the same result. The data are consistent with the hypothesis that at a low, but finite, probability, fully functional transcription complexes can form on a given active template in the absence of enhancer DNA. Enhancers seem to increase the number of active templates. Subcloning experiments suggest that these transcription complexes can be surprisingly stable.