The Hematopoietic Stem and Progenitor Cell Cistrome: GATA Factor-Dependent cis-Regulatory Mechanisms.

Transcriptional regulators mediate the genesis and function of the hematopoietic system by binding complex ensembles of cis-regulatory elements to establish genetic networks. While thousands to millions of any given cis-element resides in a genome, how transcriptional regulators select these sites and how site attributes dictate functional output is not well understood. An instructive system to address this problem involves the GATA family of transcription factors that control vital developmental and physiological processes and are linked to multiple human pathologies. Although GATA factors bind DNA motifs harboring the sequence GATA, only a very small subset of these abundant motifs are occupied in genomes. Mechanistic studies revealed a unique configuration of a GATA factor-regulated cis-element consisting of an E-box and a downstream GATA motif separated by a short DNA spacer. GATA-1- or GATA-2-containing multiprotein complexes at these composite elements control transcription of genes critical for hematopoietic stem cell emergence in the mammalian embryo, hematopoietic progenitor cell regulation, and erythroid cell maturation. Other constituents of the complex include the basic helix-loop-loop transcription factor Scl/TAL1, its heterodimeric partner E2A, and the Lim domain proteins LMO2 and LDB1. This chapter reviews the structure/function of E-box-GATA composite cis-elements, which collectively constitute an important sector of the hematopoietic stem and progenitor cell cistrome.

Navigating Transcriptional Coregulator Ensembles to Establish Genetic Networks: A GATA Factor Perspective.

Complex developmental programs require orchestration of intrinsic and extrinsic signals to control cell proliferation, differentiation, and survival. Master regulatory transcription factors are vital components of the machinery that transduce these stimuli into cellular responses. This is exemplified by the GATA family of transcription factors that establish cell type-specific genetic networks and control the development and homeostasis of systems including blood, vascular, adipose, and cardiac. Dysregulated GATA factor activity/expression underlies anemia, immunodeficiency, myelodysplastic syndrome, and leukemia. Parameters governing the capacity of a GATA factor expressed in multiple cell types to generate cell type-specific transcriptomes include selective coregulator usage and target gene-specific chromatin states. As knowledge of GATA-1 mechanisms in erythroid cells constitutes a solid foundation, we will focus predominantly on GATA-1, while highlighting principles that can be extrapolated to other master regulators. GATA-1 interacts with ubiquitous and lineage-restricted transcription factors, chromatin modifying/remodeling enzymes, and other coregulators to activate or repress transcription and to maintain preexisting transcriptional states. Major unresolved issues include: how does a GATA factor selectively utilize diverse coregulators; do distinct epigenetic landscapes and nuclear microenvironments of target genes dictate coregulator requirements; and do gene cohorts controlled by a common coregulator ensemble function in common pathways. This review will consider these issues in the context of GATA factor-regulated hematopoiesis and from a broader perspective.

Crosstalk between PKCα and Notch-4 in endocrine-resistant breast cancer cells.

The Notch pathway is functionally important in breast cancer. Notch-1 has been reported to maintain an estrogen-independent phenotype in estrogen receptor α (ERα)+ breast cancer cells. Notch-4 expression correlates with Ki67. Notch-4 also plays a key role in breast cancer stem-like cells. Estrogen-independent breast cancer cell lines have higher Notch activity than estrogen-dependent lines. Protein kinase Cα (PKCα) overexpression is common in endocrine-resistant breast cancers and promotes tamoxifen (TAM)-resistant growth in breast cancer cell lines. We tested whether PKCα overexpression affects Notch activity and whether Notch signaling contributes to endocrine resistance in PKCα-overexpressing breast cancer cells.Analysis of published microarray data from ERα+ breast carcinomas shows that PKCα expression correlates strongly with Notch-4. Real-time reverse transcription PCR and immunohistochemistry on archival specimens confirmed this finding. In a PKCα-overexpressing, TAM-resistant T47D model, PKCα selectively increases Notch-4, but not Notch-1, expression in vitro and in vivo. This effect is mediated by activator protein-1 (AP-1) occupancy of the Notch-4 promoter. Notch-4 knockdown inhibits estrogen-independent growth of PKCα-overexpressing T47D cells, whereas Notch-4IC expression stimulates it. Gene expression profiling shows that multiple genes and pathways associated with endocrine resistance are induced in Notch-4IC- and PKCα-expressing T47D cells. In PKCα-overexpressing T47D xenografts, an orally active γ-secretase inhibitor at clinically relevant doses significantly decreased estrogen-independent tumor growth, alone and in combination with TAM. In conclusion, PKCα overexpression induces Notch-4 through AP-1. Notch-4 promotes estrogen-independent, TAM-resistant growth and activates multiple pathways connected with endocrine resistance and chemoresistance. Notch inhibitors should be clinically evaluated in PKCα- and Notch-4-overexpressing, endocrine-resistant breast cancers.

Transcriptional control of erythropoiesis: emerging mechanisms and principles.

Transcriptional networks orchestrate fundamental biological processes, including hematopoiesis, in which hematopoietic stem cells progressively differentiate into specific progenitors cells, which in turn give rise to the diverse blood cell types. Whereas transcription factors recruit coregulators to chromatin, leading to targeted chromatin modification and recruitment of the transcriptional machinery, many questions remain unanswered regarding the underlying molecular mechanisms. Furthermore, how diverse cell type-specific transcription factors function cooperatively or antagonistically in distinct cellular contexts is poorly understood, especially since genes in higher eukaryotes commonly encompass broad chromosomal regions (100 kb and more) and are littered with dispersed regulatory sequences. In this article, we describe an important set of transcription factors and coregulators that control erythropoiesis and highlight emerging transcriptional mechanisms and principles. It is not our intent to comprehensively survey all factors implicated in the transcriptional control of erythropoiesis, but rather to underscore specific mechanisms, which have potential to be broadly relevant to transcriptional control in diverse systems.

Distinct mechanisms control RNA polymerase II recruitment to a tissue-specific locus control region and a downstream promoter.

Histone acetylation precedes activation of many genes. However, the establishment and consequences of long-range acetylation patterns are poorly understood. To define molecular determinants of the developmentally dynamic histone acetylation pattern of the beta-globin locus, we compared acetylation of the locus in MEL and CB3 erythroleukemia cells. CB3 cells lack the beta-globin locus control region (LCR) binding protein p45/NF-E2. We found that p45/NF-E2 was required for histone hyperacetylation at adult beta-globin promoters approximately 50 kilobases downstream of the LCR, but not at the LCR. Surprisingly, RNA polymerase II associated with the LCR in a p45/NF-E2-independent manner, while its recruitment to the promoter required p45/NF-E2. We propose that polymerase accesses the LCR and p45/NF-E2 induces long-range transfer of polymerase to the promoter, resulting in transcriptional activation.

Histone acetylation beyond promoters: long-range acetylation patterns in the chromatin world.

Histone acetylation is an important regulatory mechanism that controls transcription and diverse nuclear processes. While great progress has been made in understanding how localized acetylation and deacetylation control promoter activity, virtually nothing is known about the consequences of acetylation throughout entire chromosomal regions. An increasing number of genes have been found to reside in large chromatin domains that are controlled by regulatory elements many kilobases away. Recent studies have shown that broad histone acetylation patterns are hallmarks of chromatin domains. The purpose of this review is to discuss how such patterns are established and their implications for regulating gene expression.

Short-chain fatty acid derivatives stimulate cell proliferation and induce STAT-5 activation.

Current chemotherapeutic and butyrate therapeutics that induce fetal hemoglobin expression generally also suppress erythropoiesis, limiting the production of cells containing fetal hemoglobin (F cells). Recently, selected short-chain fatty acid derivatives (SCFADs) were identified that induce endogenous gamma-globin expression in K562 cells and human burst-forming units-erythroid and that increase proliferation of human erythroid progenitors and a multilineage interleukin-3-dependent hematopoietic cell line. In this report, gamma-globin inducibility by these SCFADs was further demonstrated in mice transgenic for the locus control region and the entire beta-globin gene locus in a yeast artificial chromosome and in 2 globin promoter-reporter assays. Conditioned media experiments strongly suggest that their proliferative activity is a direct effect of the test compounds. Investigation of potential mechanisms of action of these SCFADs demonstrates that these compounds induce prolonged expression of the growth-promoting genes c-myb and c-myc. Both butyrate and specific growth-stimulatory SCFADs induced prolonged signal transducer and activator of transcription (STAT)-5 phosphorylation and activation, and c-cis expression, persisting for more than 120 minutes, whereas with IL-3 alone phosphorylation disappeared within minutes. In contrast to butyrate treatment, the growth-stimulating SCFADs did not result in bulk histone H4 hyperacetylation or induction of p21(Waf/Cip), which mediates the suppression of cellular growth by butyrate. These findings suggest that the absence of bulk histone hyperacetylation and p21 induction, but prolonged induction of cis, myb, myc, and STAT-5 activation, contribute to the cellular proliferation induced by selected SCFADs.

Developmentally dynamic histone acetylation pattern of a tissue-specific chromatin domain.

We have defined the histone acetylation pattern of the endogenous murine beta-globin domain, which contains the erythroidspecific beta-globin genes. The beta-globin locus control region (LCR) and transcriptionally active promoters were enriched in acetylated histones in fetal liver relative to fetal brain, whereas the inactive promoters were hypoacetylated. In contrast, the LCR and both active and inactive promoters were hyperacetylated in yolk sac. Hypersensitive site two of the LCR was also hyperacetylated in murine embryonic stem cells, whereas beta-globin promoters were hypoacetylated. Thus, the acetylation pattern varied at different developmental stages. Histone deacetylase inhibition selectively increased acetylation at a hypoacetylated promoter in fetal liver, suggesting that active deacetylation contributes to silencing of promoters. We propose that dynamic histone acetylation and deacetylation play an important role in the developmental control of beta-globin gene expression.

A HECT domain ubiquitin ligase closely related to the mammalian protein WWP1 is essential for Caenorhabditis elegans embryogenesis.

The highly conserved ubiquitin/proteasome pathway controls the degradation of many critical regulatory proteins. Proteins are posttranslationally conjugated to ubiquitin through a concerted set of reactions involving activating (E1), conjugating (E2), and ligase (E3) enzymes. Ubiquitination targets proteins for proteolysis via the proteasome and may regulate protein function independent of proteolysis. We describe the cloning and functional analysis of new members of the HECT domain family of E3 ubiquitin ligases. Murine Wwp1 encoded a broadly expressed protein containing a C2 domain, four WW domains, and a catalytic HECT domain. A Caenorhabditis elegans gene was cloned encoding a HECT domain protein (CeWWP1), which was highly homologous to murine and human WWP1. Disruption of CeWwp1 via RNA interference yielded an embryonic lethal phenotype, despite the presence of at least six additional C. elegans genes encoding HECT domain proteins. The embryonic lethality was characterized by grossly abnormal morphogenesis during late embryogenesis, despite normal proliferation early in embryogenesis. CeWWP1 must therefore have unique and nonredundant functions critical for embryogenesis.

Direct interaction of NF-E2 with hypersensitive site 2 of the beta-globin locus control region in living cells.

The human beta-globin locus control region (LCR) confers high-level, tissue-specific expression to the beta-globin genes. Tandem Maf recognition elements (MAREs) within the hypersensitive site 2 (HS2) subregion of the LCR are important for the strong enhancer activity of the LCR. Multiple proteins are capable of interacting with these sites in vitro, including the erythroid cell- and megakaryocyte-specific transcription factor, NF-E2. The importance of NF-E2 for beta-globin gene expression is evident in murine erythroleukemia cells lacking the p45 subunit of NF-E2. These CB3 cells have a severe defect in alpha- and beta-globin gene transcription, which can be restored by expression of NF-E2. However, mice nullizygous for p45 express nearly normal levels of beta-globin. Thus, either a redundant factor(s) exists in mice that can functionally replace NF-E2, or NF-E2 does not function through the LCR to regulate beta-globin gene expression. To address this issue, we asked whether NF-E2 binds directly to the tandem MAREs of HS2 in intact cells. Using a chromatin immunoprecipitation assay, we provide evidence for NF-E2 binding directly and specifically to HS2 in living erythroleukemia cells and in mouse fetal liver. The specific immunoisolation of HS2 sequences was dependent on the presence of p45 and on intact MAREs within HS2. These results support a direct role for NF-E2 in the regulation of beta-globin gene expression through activation of the LCR.

Suppression of erythroid but not megakaryocytic differentiation of human K562 erythroleukemic cells by notch-1.

The Notch signal transduction pathway is a highly conserved regulatory system that controls multiple developmental processes. We have established an erythroleukemia cell model to study how Notch regulates cell fate and erythroleukemic cell differentiation. K562 and HEL cells expressed the Notch-1 receptor and the Notch ligand Jagged-1. The stable expression of the constitutively active intracellular domain of Notch-1 (NIC-1) in K562 cells inhibited erythroid without affecting megakaryocytic maturation. Expression of antisense Notch-1 induced spontaneous erythroid maturation. Suppression of erythroid maturation by NIC-1 did not result from down-regulation of GATA-1 and TAL-1, transcription factors necessary for erythroid differentiation. Microarray gene expression analysis identified genes activated during erythroid maturation, and NIC-1 disrupted the maturation-dependent changes in the expression of these genes. These results show that NIC-1 alters the pattern of gene expression in K562 cells leading to a block in erythroid maturation and therefore suggest that Notch signaling may control the developmental potential of normal and malignant erythroid progenitor cells.

Linking Notch signaling, chromatin remodeling, and T-cell leukemogenesis.

Intercellular communication that controls the developmental fate of multipotent cells is commonly mediated by the Notch family of transmembrane receptors. Specific transmembrane ligands activate Notch receptors on neighboring cells inducing the proteolytic liberation and nuclear translocation of the intracellular domain of Notch (N(IC)). Nuclear N(IC) associates with a transcriptional repressor known as C-promoter binding factor/RBP-J kappa, suppressor of hairless, or LAG-1, converting it from a repressor into an activator. Through physical interactions with chromatin remodeling enzymes and potentially with components of the transcriptional machinery, N(IC) activates target genes that mediate cell fate decisions. As Notch1 is disrupted via a chromosomal translocation in a subset of human T-cell leukemia, leading to a truncated polypeptide resembling N(IC), deregulated chromatin remodeling and transcription may fuel uncontrolled cell proliferation in this hematopoietic malignancy. This review summarizes the mechanics of Notch signaling and focuses on prospective molecular mechanisms for how constitutively active Notch might derail nuclear processes as an initiating step in T-cell leukemogenesis. J. Cell. Biochem. Suppl. 35:46-53, 2000.

Requirement of an E1A-sensitive coactivator for long-range transactivation by the beta-globin locus control region.

Four erythroid-specific DNase I-hypersensitive sites at the 5'-end of the beta-globin locus confer high-level transcription to the beta-globin genes. To identify coactivators that mediate long-range transactivation by this locus control region (LCR), we assessed the influence of E1A, an inhibitor of the CBP/p300 histone acetylase, on LCR function. E1A strongly inhibited transactivation of Agamma- and beta-globin promoters by the HS2, HS2-HS3, and HS1-HS4 subregions of the LCR in human K562 and mouse erythroleukemia cells. Short- and long-range transactivation mediated by the LCR were equally sensitive to E1A. The E1A sensitivity was apparent in transient and stable transfection assays, and E1A inhibited expression of the endogenous gamma-globin genes. Only sites for NF-E2 within HS2 were required for E1A sensitivity in K562 cells, and E1A abolished transactivation mediated by the activation domain of NF-E2. E1A mutants defective in CBP/p300 binding only weakly inhibited HS2-mediated transactivation, whereas a mutant defective in retinoblastoma protein binding strongly inhibited transactivation. Expression of CBP/p300 potentiated HS2-mediated transactivation. Moreover, expression of GAL4-CBP strongly increased transactivation of a reporter containing HS2 with a GAL4 site substituted for the NF-E2 sites. Thus, we propose that a CBP/p300-containing coactivator complex is the E1A-sensitive factor important for LCR function.

Enhancement of beta-globin locus control region-mediated transactivation by mitogen-activated protein kinases through stochastic and graded mechanisms.

Activation of the mitogen-activated protein kinase (MAPK) pathway enhances long-range transactivation by the beta-globin locus control region (LCR) (W. K. Versaw, V. Blank, N. M. Andrews, and E. H. Bresnick, Proc. Natl. Acad. Sci. USA 95:8756-8760, 1998). The enhancement requires tandem recognition sites for the hematopoietic transcription factor NF-E2 within the hypersensitive site 2 (HS2) subregion of the LCR. To distinguish between mechanisms of induction involving the activation of silent promoters or the increased efficacy of active promoters, we analyzed basal and MAPK-stimulated HS2 enhancer activity in single, living cells. K562 erythroleukemia cells stably transfected with constructs containing the human Agamma-globin promoter linked to an enhanced green fluorescent protein (EGFP) reporter, with or without HS2, were analyzed for EGFP expression by flow cytometry. When most cells in a population expressed EGFP, MAPK augmented the activity of active promoters. However, under conditions of silencing, in which cells reverted to a state with no measurable EGFP expression, MAPK activated silent promoters. Furthermore, studies of populations of EGFP-expressing and non-EGFP-expressing cells isolated by flow cytometry showed that MAPK activation converted nonexpressing cells into expressing cells and increased expression in expressing cells. These results support a model in which MAPK elicits both graded and stochastic responses to increase HS2-mediated transactivation from single chromatin templates.

Physical and functional interactions between the transactivation domain of the hematopoietic transcription factor NF-E2 and WW domains.

Tandem binding sites for the hematopoietic transcription factor NF-E2 in the beta-globin locus control region activate high-level beta-globin gene expression in transgenic mice. NF-E2 is a heterodimer consisting of a hematopoietic subunit p45 and a ubiquitous subunit p18. Gavva et al. [Gavva, N. R., Gavva, R., Ermekova, K., Sudol, M., and Shen, J. C. (1997) J. Biol. Chem. 272, 24105-24108] reported that human p45 contains a PPXY motif that binds WW domains. We show that murine NF-E2, which contains two PPXY motifs (PPXY-1 and -2) within its transactivation domain, differentially interacted with nine GST-WW domain fusion proteins. Quantitative analysis revealed high-affinity binding (KD = 5.7 nM) of p45 to a WW domain from a novel human ubiquitin ligase homologue (WWP1) expressed in hematopoietic tissues. The amino-terminal WW domain of WWP1 formed a multimeric complex with DNA-bound NF-E2. A WWP1 ligand peptide, isolated by phage display, and a peptide spanning PPXY-1 inhibited p45 binding, whereas an SH3 domain-interacting peptide and a peptide spanning PPXY-2 did not. Mutation of PPXY-1, but not PPXY-2, inhibited the transactivation function of NF-E2, providing support for the hypothesis that WW domain interactions are important for NF-E2-mediated transactivation.

Identity of the beta-globin locus control region binding protein HS2NF5 as the mammalian homolog of the notch-regulated transcription factor suppressor of hairless.

Previously, we characterized a DNA-binding protein, HS2NF5, that bound tightly to a conserved region within hypersensitive site 2 (HS2) of the human beta-globin locus control region (LCR) (Lam, L. T. , and Bresnick, E. H. (1996) J. Biol. Chem. 271, 32421-32429). The beta-globin LCR controls the chromatin structure, transcription, and replication of the beta-globin genes. We have now purified HS2NF5 to near-homogeneity from fetal bovine thymus. Two polypeptides of 56 and 61 kDa copurified with the DNA binding activity. The two proteins bound to the LCR recognition site with an affinity (3.1 nM) and specificity similar to mouse erythroleukemia cell HS2NF5. The amino acid sequences of tryptic peptides of purified HS2NF5 revealed it to be identical to the murine homolog of the suppressor of hairless transcription factor, also known as recombination signal binding protein Jkappa or C promoter binding factor 1 (CBF1). The CBF1 site within HS2 resides near sites for hematopoietic regulators such as GATA-1, NF-E2, and TAL1. An additional conserved, high affinity CBF1 site was localized within HS4 of the LCR. As CBF1 is a downstream target of the Notch signaling pathway, we propose that Notch may modulate LCR activity during hematopoiesis.

Sequence determinants of DNA binding by the hematopoietic helix-loop-helix transcription factor TAL1: importance of sequences flanking the E-box core.

TAL1 is a helix-loop-helix transcription factor that is essential for hematopoiesis. In vitro DNA binding site selection experiments have previously identified the preferred binding site for TAL1 heterodimers as AACAGATGGT. TAL1 homodimers do not bind DNA with significant affinity. A subset of other E-box sequences is also bound by TAL1 heterodimers. Here, we present an analysis of TAL1 heterodimer DNA binding specificity, using E-boxes derived from genomic clones, which were isolated by immunoadsorption of K562 erythroleukemia cell chromatin with a TAL1 antibody. We show that TAL1 heterodimer binding to a CAGATG E-box is strongly modulated by nucleotides flanking the E-box. A 10 base pair element consisting of the CAGATG E-box and two flanking nucleotides in both the 5' and 3' direction is sufficient for high-affinity binding. Certain mutations of nucleotides in either the 5' (-1 and -2) or 3' (+1 and +2) direction strongly inhibit binding. The importance of flanking nucleotides also exists in the context of nonpreferred E-boxes recognized by TAL1 heterodimers. Although there are no known target genes for TAL1, the regulatory regions of several genes involved in hematopoiesis contain the preferred E-box CAGATG. However, based on our results, the E-boxes in these potential target genes contain flanking sequences that would be expected to significantly reduce TAL1 heterodimer binding in vitro. Thus, additional stabilizing forces, such as protein-protein interactions between TAL1 heterodimers and accessory factors, may be required to confer high-affinity TAL1 heterodimer binding to such sequences.

Mitogen-activated protein kinases enhance long-range activation by the beta-globin locus control region.

The human beta-globin locus control region (LCR), which consists of four erythroid-specific DNase I hypersensitive sites (HS1-HS4), functions over a long distance to control the transcription, chromatin structure, and replication of the beta-globin genes. We have used stable transfection assays to show that activation of the mitogen-activated protein (MAP) kinase pathway by low concentrations of the phorbol ester phorbol 12-tetradecanoate 13-acetate (TPA) induces enhancer activity of the LCR subregion HS2, but not HS3. Although HS2 enhancer activity is diminished with increasing distance from the promoter, the relative level of induction by TPA is independent of HS2-promoter distance. Mutation of cis-elements within HS2 reveals that the tandem-binding sites for the hematopoietic-specific transcription factor NF-E2 are required for induction by TPA, and induction is conferred by expressing NF-E2 in an NF-E2-null cell line. These results show that MAP kinases target factors functioning through the NF-E2 sites to enhance long-range transactivation by the LCR.

Synergism between hypersensitive sites confers long-range gene activation by the beta-globin locus control region.

The human beta-globin locus control region (LCR) consists of four erythroid-specific DNaseI hypersensitive sites (HSs) at the 5' end of the beta-globin cluster. The LCR functions over a long distance on chromosome 11 to regulate transcription and replication of the beta-globin genes. To determine whether the HSs function independently or as an integrated unit, we analyzed the requirements for long-range transcriptional activation. If the HSs function independently, individual HSs would be expected to have long-range activity. In contrast, if long-range activity requires multiple HSs, individual HSs would have a limited functional distance. HS2, HS3, and a miniLCR containing multiple HSs, were separated from a gamma-globin promoter by fragments of phage lambda DNA. After stable transfection into K562 cells, HS2 had strong enhancer activity, but only when positioned close to the promoter. HS3 also had strong enhancer activity, although it was weaker than HS2 and more sensitive to the spacer DNA. The miniLCR had the strongest enhancer activity and functioned even at a distance of 7.3 kb. A model is proposed in which synergistic interactions between HSs confer long-range activation by creating a stable LCR nucleoprotein structure, which is competent for recruiting chromatin-modifying enzymes. These enzymes would mediate the well-characterized activity of the LCR to modulate chromatin structure.

Human histone acetyltransferase GCN5 exists in a stable macromolecular complex lacking the adapter ADA2.

Acetylation of core histones is an important regulatory step in transcriptional activation from chromatin templates. The yeast transcriptional coactivator protein GCN5 was recently shown to be a nuclear histone acetyltransferase (HAT). Genetic and biochemical studies in yeast suggest that GCN5 functions with the adapter proteins ADA1, ADA2, ADA3, and ADA5 in a heteromeric complex. We have established conditions for chromatographic fractionation of HATs and ADA2 from human K562 erythroleukemia cells. Gel-filtration chromatography revealed two populations of GCN5 with Stokes' radii of 67 and 33 A, consistent with a large macromolecular complex and a monomer, respectively. The GCN5-related HAT, PCAF, was resolved as a stable complex with a Stokes' radius of 74 A. The HAT complexes were resistant to 0.3 M NaCl and DNase I. ADA2 was characterized by a Stokes' radius of 35 A, consistent with a monomer. Thus, in contrast to the stable GCN5-adapter complex in yeast, human GCN5 and ADA2 are not stably associated with each other. The implications of this result are discussed vis-a-vis the mechanism of recruitment of GCN5 to regulatory regions of genes.