DNA sequences involved in transcriptional regulation of the mouse beta-globin promoter in murine erythroleukemia cells

Enhanced expression of recombinant human cyclooxygenase 1 from stably-transfected Drosophila melanogaster S2 cells by dimethyl sulfoxide is mediated by up-regulation of nitric oxide synthase and transcription factor Kr-h1

Recombinant human cyclooxygenase 1 (COX-1) was expressed from stably-transfected Drosophila melanogaster S2 (S2) cells. DMSO improved the expression of recombinant COX-1 by 180 %. DMSO increased the expression of nitric oxide synthase (NOS) at both the RNA and protein levels; NOS expression was closely correlated with the synthesis of recombinant COX-1 mRNA in stably-transfected S2 cells. DMSO also induced the gene encoding Kr-h1 which binds to the CACCC element of the metallothionein promoter to enhance the expression of recombinant COX-1. Therefore, DMSO improves the expression of recombinant COX-1 via NOS and/or the transcription factor Kr-h1.

Erythroid Kruppel-like factor (EKLF) is recruited to the gamma-globin gene promoter as a co-activator and is required for gamma-globin gene induction by short-chain fatty acid derivatives

OBJECTIVES

The erythroid Kruppel-like factor (EKLF) is an essential transcription factor for beta-type globin gene switching, and specifically activates transcription of the adult beta-globin gene promoter. We sought to determine if EKLF is also required for activation of the gamma-globin gene by short-chain fatty acid (SCFA) derivatives, which are now entering clinical trials.

METHODS

The functional and physical interaction of EKLF and co-regulatory molecules with the endogenous human globin gene promoters was studied in primary human erythroid progenitors and cell lines, using chromatin immunoprecipitation (ChIP) assays and genetic manipulation of the levels of EKLF and co-regulators.

RESULTS AND CONCLUSIONS

Knockdown of EKLF prevents SCFA-induced expression of the gamma-globin promoter in a stably expressed microLCRbeta(pr)R(luc) (A)gamma(pr)F(luc) cassette, and prevents induction of the endogenous gamma-globin gene in primary human erythroid progenitors. EKLF is actively recruited to endogenous gamma-globin gene promoters after exposure of primary human erythroid progenitors, and murine hematopoietic cell lines, to SCFA derivatives. The core ATPase BRG1 subunit of the human SWI/WNF complex, a ubiquitous multimeric complex that regulates gene expression by remodeling nucleosomal structure, is also required for gamma-globin gene induction by SCFA derivatives. BRG1 is actively recruited to the endogenous gamma-globin promoter of primary human erythroid progenitors by exposure to SCFA derivatives, and this recruitment is dependent upon the presence of EKLF. These findings demonstrate that EKLF, and the co-activator BRG1, previously demonstrated to be required for definitive or adult erythropoietic patterns of globin gene expression, are co-opted by SCFA derivatives to activate the fetal globin genes.

Functional human endogenous retroviral LTR transcription start sites are located between the R and U5 regions

Human endogenous retroviruses (HERVs) occupy about 5% of human DNA and are thought to be remnants of ancient retroviral infections of human ancestors' germ cells. HERVs can modify expression of host cell genes through their cis-regulatory elements concentrated in their long terminal repeats (LTRs). Although numerous HERV-related RNAs were identified in the human transcriptome, for most of them, it remains unclear whether they are LTR-promoted or read-through products initiated from neighboring genomic promoters. Here, we describe mapping of transcriptional start sites within solitary and proviral LTRs of the HERV-K (HML-2) human-specific subfamily of endogenous retroviruses. Surprisingly, the transcription was initiated predominantly from the very 3' termini of the LTR R regions. The data presented here may shed light on adaptive coevolution of human endogenous retroviruses with their host cells.

A global role for EKLF in definitive and primitive erythropoiesis

Erythroid Kruppel-like factor (EKLF, KLF1) plays an important role in definitive erythropoiesis and beta-globin gene regulation but failure to rectify lethal fetal anemia upon correction of globin chain imbalance suggested additional critical EKLF target genes. We employed expression profiling of EKLF-null fetal liver and EKLF-null erythroid cell lines containing an inducible EKLF-estrogen receptor (EKLF-ER) fusion construct to search for such targets. An overlapping list of EKLF-regulated genes from the 2 systems included alpha-hemoglobin stabilizing protein (AHSP), cytoskeletal proteins, hemesynthesis enzymes, transcription factors, and blood group antigens. One EKLF target gene, dematin, which encodes an erythrocyte cytoskeletal protein (band 4.9), contains several phylogenetically conserved consensus CACC motifs predicted to bind EKLF. Chromatin immunoprecipitation demonstrated in vivo EKLF occupancy at these sites and promoter reporter assays showed that EKLF activates gene transcription through these DNA elements. Furthermore, investigation of EKLF target genes in the yolk sac led to the discovery of unexpected additional defects in the embryonic red cell membrane and cytoskeleton. In short, EKLF regulates global erythroid gene expression that is critical for the development of primitive and definitive red cells.

C/EBPδ and C/EBPγ bind the CCAAT-box in the human β-globin promoter and modulate the activity of the CACC-box binding protein, EKLF

Developmental- and tissue-specific expression of globin genes is mediated by a few key elements within the proximal promoter of each gene. DNA-binding assays previously identified NF-Y, GATA-1, C/EBPbeta and C/EBPgamma as candidate regulators of beta-globin transcription via the CCAAT-box, a promoter element situated between CACC- and TATA-boxes. We have identified C/EBPdelta as an additional beta-globin CCAAT-box binding protein. In reporter assays, we show that C/EBPdelta can co-operate with EKLF, a CACC-box binding protein, to activate the beta-globin promoter, whereas C/EBPgamma inhibits the transcriptional activity of EKLF in this assay.

NF-Y organizes the gamma-globin CCAAT boxes region

The CCAAT-binding activator NF-Y is formed by three evolutionary conserved subunits, two of which contain putative histone-like domains. We investigated NF-Y binding to all CCAAT boxes of globin promoters in direct binding, competition, and supershift electrophoretic mobility shift assay; we found that the alpha, zeta, and proximal gamma CCAAT boxes of human and the prosimian Galago bind avidly, and distal gamma CCAAT boxes have intermediate affinity, whereas the epsilon and beta sequences bind NF-Y very poorly. We developed an efficient in vitro transcription system from erythroid K562 cells and established that both the distal and the proximal CCAAT boxes are important for optimal gamma-globin promoter activity. Surprisingly, NF-Y binding to a mutated distal CCAAT box (a C to T at position -114) is remarkably increased upon occupancy of the high affinity proximal element, located 27 base pairs away. Shortening the distance between the two CCAAT boxes progressively prevents simultaneous CCAAT binding, indicating that NF-Y interacts in a mutually exclusive way with CCAAT boxes closer than 24 base pairs apart. A combination of circular permutation and phasing analysis proved that (i) NF-Y-induced angles of the two gamma-globin CCAAT boxes have similar amplitudes; (ii) occupancy of the two CCAAT boxes leads to compensatory distortions; (iii) the two NF-Y bends are spatially oriented with combined twisting angles of about 100 degrees. Interestingly, such distortions are reminiscent of core histone-DNA interactions. We conclude that NF-Y binding imposes a high level of functionally important coordinate organization to the gamma-globin promoter.

The keratinocyte-specific Epstein-Barr virus ED-L2 promoter is regulated by phorbol 12-myristate 13-acetate through two cis-regulatory elements containing E-box and Krüppel-like factor motifs

We previously employed 782 base pairs of the Epstein-Barr virus ED-L2 early lytic cycle promoter in a transgenic mouse model to target cyclin D1 to the stratified squamous epithelium of the tongue and esophagus. This promoter is located 5' to the transcriptional start site of a short open reading frame BNLF-2A and is immediately 3' to the BNLF-1 (LMP-1 oncogene) open reading frame. We studied transcriptional regulation of the ED-L2 promoter by phorbol 12-myristate 13-acetate (PMA) as a means of understanding the tissue specificity of this promoter. The transcriptional activity of the ED-L2 promoter was stimulated 40-fold by PMA and could be blocked with the compound H7 through antagonism of protein kinase C. 5' deletion analysis of the 782-base pair promoter demonstrated that the sequences necessary for PMA-stimulated trans-activation were located in two separate cis-regulatory regions of the promoter: -187 to -164 and -144 to -114 base pairs from the transcription start site of BNLF-2A. Importantly, mutation of critical base pairs in each region was sufficient to abolish PMA-stimulated trans-activation in the native ED-L2 promoter. Region -187 to -164 contains a CACCTG (E-box) motif, and region -144 to -114 contains a CACACCC motif. Both of these motifs are necessary for trans-activation by PMA. These regions do not, however, demonstrate enhancer characteristics when tested in a heterologous minimal promoter system. Variations of the CACACCC motif are found in other keratinocyte-specific promoters, as well as in the DNA binding motifs of the Krüppel-like family of transcription factors. Electrophoretic mobility shift assays with specific competitors and factor-specific antibody supershift assays demonstrated that one complex binding the -187 to -164 region containing the CACCTG nucleotides has characteristics of the helix-loop-helix protein upstream stimulatory factor, whereas a factor binding the CACACCC motif may be a member of the Krüppel-like family. These experiments show how ubiquitous and tissue-specific transcription factors induced by PMA regulate the ED-L2 promoter in squamous epithelial cells.

A CACCC box-like cis-regulatory element of the Epstein-Barr virus ED-L2 promoter interacts with a novel transcriptional factor in tissue-specific squamous epithelia

The Epstein-Barr (EBV) virus induces a lytic state after infecting epithelial cells. Subsequently, there is infection of B lymphocytes with two types of cycles, latent and lytic. Apart from linkage of the EBV latent membrane protein-1 (LMP-1) with benign and malignant conditions of squamous epithelial cells, little is known about other EBV gene products that may be important in these processes as well as cellular transcriptional factors that regulate EBV gene expression in these epithelial cells. The EBV ED-L2 promoter, an early lytic cycle promoter, is located upstream of a transcription start site for a short open reading frame designated BNLF2 and just downstream of the BNLF1 (LMP-1) open reading frame. We have previously used the EBV ED-L2 promoter to target oncogenes in transgenic mice, resulting in tissue-specific expression in the tongue, esophagus, forestomach, and skin, all sharing stratifying squamous epithelia, alternatively called keratinocytes. In the present study, we have functionally dissected the ED-L2 promoter by making deletion constructs fused to the luciferase reporter gene with transient transfections into squamous and nonsquamous epithelial cell lines as well as B lymphocytes. A CACCC box-like cis-regulatory element has been identified that is located between -218 and -187 base pairs of the ED-L2 promoter that confers significant promoter activity only in squamous epithelial cells. This cis-regulatory element is active in a heterologous minimal herpes simplex virus thymidine kinase promoter reporter gene construct when transfected into squamous epithelial cells but not in nonsquamous epithelial cells. DNA gel mobility shift assays have led to the identification of DNA-protein complexes that bind the CACCC box-like element. One of these proteins is a novel transcriptional factor that is uniquely active in stratified squamous epithelial cells, designated as keratinocyte specific factor (KSF). KSF may be related to Sp1 but appears to be distinct from Sp1. In addition, KSF may interact with related or identical cis-regulatory elements found in human papillomavirus-11 E6 and cytokeratin K3 promoters that are active in keratinocytes. In aggregate, KSF may be important in the transcriptional regulation of viral and eukaryotic genes in keratinocytes.

Cruciform structures and functions

In this paper, a structure-function analysis of B-DNA self-fitting is reviewed in the light of recent oligonucleotide crystal structures. Their crystal packings provided a high-resolution view of B-DNA helices closely and specifically fitted by groove-backbone interaction, a natural and biologically relevant manner to assemble B-DNA helices. In revealing that new properties of the DNA molecule emerge during condensation, these crystallographic studies have pointed to the biological importance of DNA—DNA interactions.

In vivo footprinting analysis of the hepatic control region of the human apolipoprotein E/C-I/C-IV/C-II gene locus

Expression of both the apolipoprotein (apo)E and apoC-I genes in the liver is specified by a 319-nucleotide hepatic control region (HCR-1) that is located 15 kilobase pairs downstream of the apoE gene and 5 kilobase pairs downstream of the apoC-I gene. In vivo footprint analysis of HCR-1 in intact nuclei revealed several liver-specific protein-binding sites that were not detectable by in vitro methods. In addition to three previously identified in vitro footprints, four in vivo footprints were identified in a region of HCR-1 that is required for directing gene expression to hepatocytes. Prominent liver-specific DNase I-hypersensitive sites were associated with these footprints. Liver-specific nuclear protein binding to these sites was confirmed by oligonucleotide gel-retention assays. The in vivo analysis also identified a cluster of nuclear protein-binding sites in the Alu family repeat segment adjacent to the domain required for liver expression. Micrococcal nuclease digestion indicated the presence of a nucleosome in the central domain of HCR-1 in liver chromatin that was in phase with the nucleosome location in tissues that did not express the transgene. These results suggest that HCR-1 functions in a highly structured chromatin environment requiring a complex interaction of liver-enriched transcription factors.

Molecular mechanisms of hemoglobin switching

The developmental regulation of the human beta-globin cluster embodies all aspects of transcriptional control of eukaryotic genes. The cis-acting sequences within the cluster, distal regulatory regions and trans-acting factors all contribute to provide stringent temporal and tissue-specific expression. This review will examine the individual regulatory mechanisms which govern globin gene expression and highlight recent advances which expand our understanding of these dynamic interactions.

A functional initiator element in the human beta-globin promoter

Core promoters are defined by the presence of either a TATA box at approximately 30 base pairs upstream of the transcriptional start site (+1) and/or an initiator element centered around the +1 site. The prevalence, function, and significance of the various combinations of core promoter elements are as yet unclear. We describe here the identification and characterization of an initiator element in the TATA-containing human beta-globin promoter. Mutagenesis of the beta-globin initiator element at positions +2/+3 and +4/+5 abrogates transcription in a heterologous construct. Interestingly, we have found a beta-globin initiator binding activity in nuclear extracts whose presence or absence correlates with function of the beta-globin initiator. Accordingly, this binding activity may be part of the machinery required for beta-globin initiator-dependent transcription. Our analysis further describes a previously uncharacterized beta-thalassemia mutation at the +1 site as a mutation that decreases beta-globin initiator activity. Finally, consistent with other initiator elements, the beta-globin initiator requires a TFIID-containing fraction for in vitro activity. Thus, the human beta-globin promoter contains an initiator element whose function, as revealed by a beta-thalassemia mutation, is of physiological relevance.

Nuclear proteins that interact with the beta maj globin promoter start to accumulate in MEL cells within 12 hours of induction and RNA copies of the promoter successfully compete their binding in vitro

The induction of differentiation in mouse erythroleukemia (MEL) cells by dimethylsulfoxide (DMSO) is characterized by increased transcription of globin genes. We have determined that DMSO treated cells increase the levels of nuclear factors capable of overall interactions with the beta maj globin promoter during the initial 24 h post induction, as measured by gel mobility analysis. Two unprocessed beta maj globin mRNA precursors, which are present in MEL cell nuclei early in differentiation, were previously shown to contain the 5' promoter flanking region, and thereby provided the nucleus with a pool of regulatory sequences in multiple RNA copies. We have studied the effect of RNA copies of the promoter region on binding interactions between DNA sequences of the beta maj globin promoter and nuclear factors that interact with these sequences. The promoter region RNA transcripts competed effectively for DNA binding proteins in vitro, while the antisense RNA from the same region did not. The most pronounced competition was observed with proteins from 12 h after DMSO induction, when the concentration of the DNA binding proteins was still increasing. Since the 'upstream' transcripts predominate at 12 h after DMSO induction, these results indicate that the promoter region transcripts may influence the equilibrium of binding between the beta maj globin promoter and the nuclear factors that bind to this region during DMSO induction.

Erythropoietin-induced transcription at the murine beta maj-globin promoter. A central role for GATA-1

Using J2E cells and the murine beta maj-globin promoter as a model, we have performed the first direct analyses of erythropoietin (EPO)-activated transcription from defined templates. The -346 to +26 beta maj promoter was shown to comprise a target for maximal activation. This included a positive role for a -346 to -107-base pair (bp) domain in J2E cells, but not in F-MEL cells. Mutagenesis of a -215-bp AGATAA element within this domain showed that this effect did not require GATA-1 binding. In contrast, a critical role for GATA-1 at a -60-bp (G)GATAG element was defined by mutagenesis (GGg-TAG and TGATAG), complementation with a synthetic TGATAA element, and the demonstrated specific binding of GATA-1. Proximal CCAAT (-75) and CACCC (-90) elements also were shown to contribute to transcriptional activation in J2E cells, yet exerted quantitatively distinct effects in the F-MEL system. Based on these results, minimal [TGATAA]4-TATA and TGATAA-CACCC-TATA promoters were constructed and assayed in each system. Remarkably, the [TGATAA]4-TATA promoter, but not the TGATAA-CACCC-TATA promoter, was induced efficiently by EPO in J2E cells, whereas the TGATAA-CACCC-TATA promoter was highly induced by Me2SO in F-MEL cells. These findings suggest that mechanisms of EPO-induced transcription in J2E cells involve GATA-1 and differ from chemically activated mechanisms studied previously in F-MEL cells. Globin induction in J2E cells was not associated with effects of EPO on levels or nuclear translocation of GATA-1. However, hemoglobinization was induced by okadaic acid, 8-Br-cAMP, and forskolin, a finding consistent with induction mechanisms that may involve modulated serine/threonine phosphorylation.

Sequence dependence of base-pair opening in a DNA dodecamer containing the CACA/GTGT sequence motif

Proton nuclear magnetic resonance spectroscopy is used to characterize the kinetics and energetics of base-pair opening in two self-complementary DNA dodecamer duplexes: [d(CGCACATGTGCG)]2 and [d(CGCAGATCTGCG)]2. The first dodecamer contains two symmetrical CACA/GTGT motifs; in the second dodecamer, each motif is interrupted by a change of the central C.G base pair to a G.C base pair. The opening rates and the equilibrium constants for formation of the open state of each base pair are obtained from the dependence of the imino proton exchange rates on the concentration of ammonia catalyst. The results indicate that the opening rates of the central three base pairs in the CACA/GTGT motif are 3-8-fold larger than the corresponding ones in the CAGA/GTCT sequence. The activation enthalpies and entropies, and the standard enthalpy and entropy changes for formation of the open state, are obtained from the temperature dependence of the opening rates and equilibrium constants, respectively. The results reveal that enthalpy/entropy compensation exists, for all base pairs in both dodecamers, in activation as well as in the equilibria between closed and open states. As a result, the opening rates and equilibrium constants for opening are maintained, in both dodecamers, within a relatively narrow range of values. Nevertheless, large sequence-induced variations are observed for the activation enthalpies and the standard enthalpy changes for opening. The A.T base pair located between the C.G base pairs in the CACA/GTGT motif has a negative enthalpy change for formation of the activated state during opening. This is the first case in which a negative activation enthalpy is observed for opening of a Watson-Crick base pair in DNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of DNA bending and flexibility using gel electrophoresis

Gel electrophoretic methods have become established as primary tools in the study and elucidation of sequence-directed curvature both in free DNA and in the operator DNA of several site-specific nucleoprotein complexes. Results using them have been generally consistent with physical methods sensitive to DNA structure and conformation in those instances where direct comparisons can be made, and in a number of cases, gel methods have provided unique information not presently available from other techniques. Two basic strategies have been used: one based upon anomalous gel mobility effects; and a second based upon cyclization properties of curved DNA. Within each of these categories, various approaches have been used, some of which can lead, in favorable cases, to quantitative estimation of bending angles. In this review, the various gel-based methods that have been used to date are critically discussed and the qualitative and quantitative information that can be obtained from them is evaluated. A number of possible structural models for DNA curvature are described and a distinction is drawn between static or fixed bending and bending due to anisotropic flexibility at specific sequence loci. The importance and roles of gel electrophoretic methods in providing experimental approaches to this question are discussed. It is suggested that both static curvature and anisotropic flexibility in operator DNA may provide much of the basis for indirect readout of sequence information by specific site-binding regulatory proteins.

CA runs increase DNA flexibility in the complex of lambda Cro protein with the OR3 site

The alternating pyrimidine-purine elements CA, CAC, and CACA are anisotropically flexible, as deduced from gel circularization assays on point mutations and single-base mismatches in the OR3 site of lambda phage alone and in the specific complex with the Cro protein. These sequences evidently promote DNA bending in the specific binding region of the complex and may also facilitate overwinding in the central nonbinding region. Effects for CACA are exceptionally large and suggest that an alternative DNA structure may occur in this element.

CCACC-binding or simian-virus-40-protein-1-binding proteins cooperate with human GATA-1 to direct erythroid-specific transcription and to mediate 5' hypersensitive site 2 sensitivity of a TATA-less promoter

Previous studies have shown that a -112 to +78 DNA fragment from the erythroid promoter of the human porphobilinogen deaminase (PBGD) gene has erythroid-specific activity. This PBGD-(-112 to +78) promoter contains a CCACC binding site (position -100), a GATA binding site (position -70) and an initiator element around the cap site. Using a cotransfection assay, we find that the human factor GATA-1 trans-activates the PBGD-(-112 to +78) promoter in non-erythroid cells. We show that, if trans-activation is abolished by mutations that destroy either the -100 CCACC binding or the -70 GATA binding sites, replacement of the -100 CCACC binding site by a simian-virus-40-protein-1 (Sp1) binding site maintains both the erythroid-specific activity of this promoter and the human GATA-1 trans-activation. Thus, human GATA-1 acts on the PBGD promoter in association with Sp1 or CCACC binding proteins. This PBGD-(-112 to +78) promoter is activated 20-fold by a cis-linked 5' hypersensitive site 2 (5'HS-2) of the human beta-globin locus control region. This activation depends on the -70 GATA and -100 CCACC or Sp1 binding sites. When a longer -714 to +78 fragment of the PBGD promoter is used, the -70 GATA mutant still displays erythroid-specific activity and is cis-activated by the 5'HS-2 enhancer, while the -100 CCACC mutant is completely inactive in the absence or in the presence of the 5'HS-2 enhancer. Thus, the -100 CCACC binding site is indispensable for the correct activity and sensitivity of the human PBGD promoter to the 5'HS-2 enhancer, whereas the -70 GATA binding site can functionally be replaced by upstream cis-acting elements.

Base-pairing shift in the major groove of (CA)n tracts by B-DNA crystal structures

the crystal packing of the B-DNA dodecamer d(ACCG-GCGCCACA).d(TGTGGCGCCGGT) is characterized by the reciprocal fit of double helices with specific base-backbone interactions in the major groove. Cooling the crystals below -10 degrees C stabilizes a new conformational state with a long-range sequence-dependent one-step shift in the major-groove base pairing. The tilt of the bases leads to the disruption of the Watson-Crick pairing in the major groove and to the formation of interactions with the 5' neighbour of their complement. This alteration propagates along the helical axis over more than half a turn. As a result, the molecular structure is normal when seen from the minor groove side and mismatched in the major groove. Comparison with a parent isomorphous dodecamer structure corresponding to the codon 10-13 of the c-Ha-ras proto-oncogene show that this new structural feature is sequence dependent and clearly favoured by (CA)n tracts. As(CA)n tracts of DNA are involved both in recombination and in transcription, this new recognition pattern should be considered in the analysis of the various processes involving the reading of the genetic information.

A differentiation stage-specific factor interacts with mouse carbonic anhydrase form I gene and a conserved sequence in mammalian beta-globin genes

We have identified in mouse erythroleukemic (MEL) cells a novel factor which binds to the 3' flanking region of beta-globin genes. Upon induction, this DNA binding factor disappears as beta-globin gene transcription increases. The factor protects 20-30 base pairs (bp) of a sequence which is tightly conserved in beta-globin genes including chicken, human, mouse and rabbit. A very similar sequence binds the factor in the mouse carbonic anhydrase form I gene, whose transcription diminishes upon induction of MEL cells. The factor, or a closely related form, is detected in nonerythroid cells and binding sites have been detected in other genes, including c-myb and immunoglobulin heavy chain-enhancer. We suggest that this factor could play a role in determining the timing of gene expression in several differentiating cell types.

Enhancer dependent expression of the chicken beta-hatching globin gene during erythroid differentiation

The activity of the chicken beta H globin gene promoter has been analysed in functional assays in both chicken and murine erythroleukaemia cells. Sequences between -251 and -146 bp, in the presence or absence of the chicken beta globin locus enhancer, strongly repress transcription in erythroid cells before and after the induction of terminal differentiation. A 50 bp sequence (-98 to -146 bp), which contains adjacent cGATA-1 and NF1 protein binding sites in vitro, and which is bound by non-histone protein in vivo, is essential for full promoter activity. Mutagenesis studies indicate that both protein binding sites are required. During terminal differentiation, both the absence of repressor and the presence of the erythroid enhancer are required for maximal promoter activity, suggesting that the beta A, beta epsilon and beta H globin gene promoters compete for the enhancer during development.

Sequence-dependent kinks induced in curved DNA

In certain curved DNA fragments without AA dinucleotides, the gel retardation anomaly associated with curvature passes through a maximum with fragment length, indicating length (and electric field) dependent structural transitions in the DNA. We suggest that thermally induced stereochemical kinks in DNA are stabilized in the gel, thus relieving the effects of curvature. These kinks are shown to occur specifically at CA/TG and TA/TA stacks. Other physical and biological evidence points to frequent structural dislocations at CA and TA steps. These reversible sequence dependent kinks may therefore represent a novel class of structural protein-DNA recognition elements.

The same nuclear proteins bind the proximal CACCC box of the human beta-globin promoter and a similar sequence in the enhancer

Using in vitro assays, we show that nuclear proteins related to the Sp1 and GT-1 factors bind to a CACCC box sequence in the human beta-globin enhancer, adjacent to binding sites for the erythroid-specific factor NFE1 and the ubiquitous factor CP1. The same proteins are known to bind to the proximal, but not to the distal, CACCC, box in the human beta-globin promoter. A C G mutation in the promoter CACCC box, known to cause beta-thalassemia, greatly decreases protein binding to the CACCC box; the same effect is obtained when this mutation is introduced into the enhancer CACCC box.