Recent evidence suggests that long-range enhancers and gene promoters are in close proximity, which might reflect the formation of chromatin loops. Here, we examined the mechanism for DNA looping at the beta-globin locus. By using chromosome conformation capture (3C), we show that the hematopoietic transcription factor GATA-1 and its cofactor FOG-1 are required for the physical interaction between the beta-globin locus control region (LCR) and the beta-major globin promoter. Kinetic studies reveal that GATA-1-induced loop formation correlates with the onset of beta-globin transcription and occurs independently of new protein synthesis. GATA-1 occupies the beta-major globin promoter normally in fetal liver erythroblasts from mice lacking the LCR, suggesting that GATA-1 binding to the promoter and LCR are independent events that occur prior to loop formation. Together, these data demonstrate that GATA-1 and FOG-1 are essential anchors for a tissue-specific chromatin loop, providing general insights into long-range enhancer function.

The Myc/Max/Mad transcription factor network is critically involved in cell behavior; however, there is relatively little information on its genomic binding sites. We have employed the DamID method to carry out global genomic mapping of the Drosophila Myc, Max, and Mad/Mnt proteins. Each protein was tethered to Escherichia coli DNA adenine-methyltransferase (Dam) permitting methylation proximal to in vivo binding sites in Kc cells. Microarray analyses of methylated DNA fragments reveals binding to multiple loci on all major Drosophila chromosomes. This approach also reveals dynamic interactions among network members as we find that increased levels of dMax influence the extent of dMyc, but not dMnt, binding. Computer analysis using the REDUCE algorithm demonstrates that binding regions correlate with the presence of E-boxes, CG repeats, and other sequence motifs. The surprisingly large number of directly bound loci (∼ 15% of coding regions) suggests that the network interacts widely with the genome. Furthermore, we employ microarray expression analysis to demonstrate that hundreds of DamID-binding loci correspond to genes whose expression is directly regulated by dMyc in larvae. These results suggest that a fundamental aspect of Max network function involves widespread binding and regulation of gene expression.[Keywords: myc; mad; Drosophila; target genes; transcription] Supplemental material is available at http://parma.fhcrc.org/AOryan.

To investigate the molecular basis of ␤-globin gene activation, we analyzed factor recruitment and histone modification at the adult ␤-globin gene in wild-type (WT)/locus control region knockout (⌬LCR) heterozygous mice and in murine erythroleukemia (MEL) cells. Although histone acetylation and methylation (Lys 4) are high before and after MEL differentiation, recruitment of the erythroid-specific activator NF-E2 to the promoter and preinitiation complex (PIC) assembly occur only after differentiation. We reported previously that targeted deletion of the LCR reduces ␤-globin gene expression to 1%-4% of WT without affecting promoter histone acetylation. Here, we report that NF-E2 is recruited equally efficiently to the adult ␤-globin promoters of the ⌬LCR and WT alleles. Moreover, the LCR deletion reduces PIC assembly only twofold, but has a dramatic effect on Ser 5 phosphorylation of RNA polymerase II and transcriptional elongation. Our results suggest at least three distinct stages in ␤-globin gene activation: (1) [Keywords: Allele-specific chromatin immunoprecipitation analysis; locus control region; ␤-globin locus; transcription elongation; NF-E2; CTD-phosphorylation] Supplemental material is available at http://www.genesdev.org. Received January 6, 2003; revised version accepted February 19, 2003. In higher eukaryotes, gene activation involves several events including chromatin opening, activator binding to regulatory regions, recruitment of basal transcription factors (TFIIA to TFIIJ) and RNA polymerase II (pol II) to the promoter [preinitiation complex (PIC) assembly], and transcription elongation. A general model has emerged in which activators function to stabilize or modulate transcription through interactions with histone acetyltransferases, as well as components of the basal transcription machinery, such as TATA binding protein (TBP), TBP-associated factors (TAFs), and TFIIB (for review, see Lemon and Tjian 2000). PIC assembly is followed by initiation and elongation, during which the Cterminal domain (CTD) of the RPB1 subunit of pol II is phosphorylated (for review, see Dahmus 1996). These steps in gene activation are often regulated by distal elements called enhancers (for review, see Blackwood and Kadonaga 1998;Martin 2001). Enhancers increase either the rate of transcription, the number of the templates engaged in transcription, or both. Studies using transgenes or in vitro templates have linked enhancer activities to various events such as PIC assembly (Kim et al. 1998;Yie et al. 1999), histone acetylation at the promoter (Madisen et al. 1998), and nuclear localization (Francastel et al. 1999). However, it is poorly understood which events in transactivation are the actual targets of long-range enhancer function at native gene loci.The murine ␤-globin gene locus is a model system for studying the molecular mechanisms of enhancer-dependent gene activation at a native locus. The locus contains embryonic and adult ␤-like globin genes that are ordered as they are expressed during development. Hig...

The mouse ␤-globin gene locus control region (LCR), located upstream of the ␤-globin gene cluster, is essential for the activated transcription of genes in the cluster. The LCR contains multiple binding sites for transactivators, including Maf-recognition elements (MAREs). However, little is known about the specific proteins that bind to these sites or the time at which they bind during erythroid differentiation. We have performed chromatin immunoprecipitation experiments to determine the recruitment of the erythroid-specific transactivator p45 NF-E2͞MafK (p18 NF-E2) heterodimer and small Maf proteins to various regions in the globin gene locus before and after the induction of murine erythroleukemia (MEL) cell differentiation. We report that, before induction, the LCR is occupied by small Maf proteins, and, on erythroid maturation, the NF-E2 complex is recruited to the LCR and the active globin promoters, even though the promoters do not contain MAREs. This differentiation-coupled recruitment of NF-E2 complex correlates with a greater than 100-fold increase in ␤-major globin transcription, but is not associated with a significant change in locus-wide histone H3 acetylation. These findings suggest that the ␤-globin gene locus exists in a constitutively open chromatin conformation before terminal differentiation, and we speculate that recruitment of NF-E2 complex to the LCR and active promoters may be a rate-limiting step in the activation of ␤-globin gene expression.