The chicken lysozyme chromatin domain contains a second, widely expressed gene

Paracrine cyclooxygenase-2 activity by macrophages drives colorectal adenoma progression in the Apc Min/+ mouse model of intestinal tumorigenesis

Genetic deletion or pharmacological inhibition of cyclooxygenase (COX)-2 abrogates intestinal adenoma development at early stages of colorectal carcinogenesis. COX-2 is localised to stromal cells (predominantly macrophages) in human and mouse intestinal adenomas. Therefore, we tested the hypothesis that paracrine Cox-2-mediated signalling from macrophages drives adenoma growth and progression in vivo in the Apc^Min/+ mouse model of intestinal tumorigenesis. Using a transgenic C57Bl/6 mouse model of Cox-2 over-expression driven by the chicken lysozyme locus (cLys-Cox-2), which directs integration site-independent, copy number-dependent transgene expression restricted to macrophages, we demonstrated that stromal macrophage Cox-2 in colorectal (but not small intestinal) adenomas from cLys-Cox-2 x Apc^Min/+ mice was associated with significantly increased tumour size (P = 0.025) and multiplicity (P = 0.025), compared with control Apc^Min/+ mice. Transgenic macrophage Cox-2 expression was associated with increased dysplasia, epithelial cell Cox-2 expression and submucosal tumour invasion, as well as increased nuclear β-catenin translocation in dysplastic epithelial cells. In vitro studies confirmed that paracrine macrophage Cox-2 signalling drives catenin-related transcription in intestinal epithelial cells. Paracrine macrophage Cox-2 activity drives growth and progression of Apc^Min/+ mouse colonic adenomas, linked to increased epithelial cell β-catenin dysregulation. Stromal cell (macrophage) gene regulation and signalling represent valid targets for chemoprevention of colorectal cancer.

Structure and function of active chromatin and DNase I hypersensitive sites

Chromatin is by its very nature a repressive environment which restricts the recruitment of transcription factors and acts as a barrier to polymerases. Therefore the complex process of gene activation must operate at two levels. In the first instance, localized chromatin decondensation and nucleosome displacement is required to make DNA accessible. Second, sequence-specific transcription factors need to recruit chromatin modifiers and remodellers to create a chromatin environment that permits the passage of polymerases. In this review I will discuss the chromatin structural changes that occur at active gene loci and at regulatory elements that exist as DNase I hypersensitive sites.

DT40 knock-out and knock-in studies determine the regions necessary and sufficient for transcription and epigenetic conversion of the chicken Ig-β gene

The chicken Ig-β locus is organized by three cell-type-specific genes and two ubiquitously expressed genes. B-cell-specific DNase I hypersensitive sites (DHS) in that locus, including three present inside the flanking gene, were grouped into six regions and deleted. The deletions decreased Ig-β mRNA content to <0.1% of that of normal DT40 cells and converted epigenetic parameters such as histone modifications, CG methylation and DNase I hypersensitivity into inactive states. Knocked-in DHS regions into knock-out cells reactivated both transcription of the Ig-β gene and epigenetic parameters. Thus, the collaboration of the scattered regulatory regions was essential and sufficient not only for B-cell-specific transcription of the Ig-β gene, but also for the conversion of epigenetic parameters. On the basis of the knock-in studies, we determined the regions involved in the conversion and maintenance of the epigenetic parameters. These scattered regulatory regions were limited in vicinity such as in an intron of the gene, in the intergenic regions and in the introns of a flanking gene.

Sp1 and Sp3 regulate transcription of the chicken GAS41 gene

The 5'-flanking region of the chicken glioma-amplified sequence (GAS) 41 gene is close to the 3' end of the lysozyme gene and contains no typical TATA box, but several GC boxes. In this study, we have localized the GAS 41 promoter to this narrow region. Electrophoretic mobility shift assays and chromatin immunoprecipitation analyses revealed that Sp1 and Sp3 bind to this promoter. Mapping by a technique of indirect end labeling demonstrated that the Sp1-binding sites contained in this region exactly co-map with two previously identified DNase I hypersensitive (HS) sites, which suggests the important role of Sp1 binding in maintaining an open chromatin structure of the GAS41 promoter. We further found that Sp1 and Sp3 strongly activate CAT expression controlled by the putative GAS41 promoter in Drosophila Schneider S2 cells and that deletion of the Sp1 sites resulted in a loss of promoter activity in chicken HD11 cells. The results indicate that transcription factors of the Sp family play an important role in the transcriptional regulation of the chicken GAS41 gene.

Early replication timing of the chicken alpha-globin gene domain correlates with its open chromatin state in cells of different lineages

The vertebrate alpha-globin gene domain is an open chromatin domain overlapping a neighboring house-keeping gene. The tissue-specific cluster of alpha-globin genes and the overlapping housekeeping gene share the same replication origin. We have studied the replication timing of chicken alpha-globin genes in cells of different lineages using the FISH-based approach and found that alpha-globin genes replicate early both in erythroid and in non-erythroid cells, i.e. regardless of their transcriptional activity. Early replication timing of chicken alpha-globin genes in cells of different lineages was in good correlation with the open chromatin configuration of the alpha-globin gene domain in both erythroid and non-erythroid cells. We propose that active transcription of the housekeeping gene overlapping the alpha-globin gene domain enables an access of Origin Recognition Complex (ORC) proteins to the replication origin resulting in early replication of alpha-globin genes even in non-erythroid cells.

The chromatin of active genes is not in a permanently open conformation

Quantitative measurements of local chromatin accessibility to DNase I in 15-day chicken embryo erythrocyte nuclei have been performed using a range of nuclease concentrations and real-time TaqMan PCR to monitor the loss of short ( approximately 80 bp) amplicons. At the beta-globin locus, well-established DNase I hypersensitive sites stand out against a background in which actively transcribed gene sequences (e.g., beta-adult and beta-hatching) are no more sensitive than the nearby constitutive heterochromatin that has previously been shown to form the 30-nm fibre structure. Similar observations were made at the lysozyme locus containing the active Gas41 gene and also at the GAPDH locus. We conclude that active genes are not continuously held in an open 'beads-on-a-string' configuration, but adopt a 30-nm-type structure most of the time. This implies that the compact nucleosomal supercoil re-forms in the wake of the polymerase complex.

The LPS-induced transcriptional upregulation of the chicken lysozyme locus involves CTCF eviction and noncoding RNA transcription

Transcription of the lysozyme gene is rapidly induced by proinflammatory stimuli such as treatment with bacterial lipopolysaccharide (LPS). Here we show that this induction involves both the relief of repression mediated by the enhancer-blocking protein CTCF that binds to a negative regulatory element at −2.4 kb, and the activation of two flanking enhancer elements. The downstream enhancer has promoter activity, and LPS stimulation initiates the transient synthesis of a noncoding RNA (LINoCR) transcribed through the −2.4 kb element. Expression of LINoCR is correlated with IKKα recruitment, histone H3 phosphoacetylation in the transcribed region, the repositioning of a nucleosome over the CTCF binding site, and, eventually, CTCF eviction. Each of these events requires transcription elongation. Our data reveal a transcription-dependent mechanism of chromatin remodeling that switches a cis-regulatory region from a repressive to an active conformation.

Molecular cloning of an invertebrate goose-type lysozyme gene from Chlamys farreri, and lytic activity of the recombinant protein

Lysozyme is a widely distributed hydrolase possessing lytic activity against bacterial peptidoglycan, which enables it to protect the host against pathogenic infection. In the present study, the cDNA of an invertebrate goose-type lysozyme (designated CFLysG) was cloned from Zhikong scallop Chlamys farreri by expressed sequence tag (EST) and rapid amplification of cDNA ends (RACE) techniques. The full-length cDNA of CFLysG consisted of 829 nucleotides with a canonical polyadenylation signal sequence AATAAA and a poly(A) tail, and an open reading frame (ORF) of 603 bp encoding a polypeptide of 200 amino acid residues with a predicted molecular weight of 21.92 kDa and theoretical isoelectric point of 7.76. The high similarity of CFLysG with goose-type (g-type) lysozymes in vertebrate indicated that CFLysG should be an invertebrate counterpart of g-type lysozyme family, which suggested that the origin of g-type lysozyme preceded the emergence of urochordates and even preceded the emergence of deuterostomes. Similar to most g-type lysozymes, CFLysG possessed all conserved features critical for the fundamental structure and function of g-type lysozymes, such as three catalytic residues (Glu 82, Asp 97, Asp 108). By Northern blot analysis, mRNA transcript of CFLysG was found to be most abundantly expressed in the tissues of gills, hepatopancreas and gonad, weakly expressed in the tissues of haemocytes and mantle, while undetectable in the adductor muscle. These results suggested that CFLysG could possess combined features of both the immune and digestive adaptive lysozymes. To gain insight into the in vitro lytic activities of CFLysG, the mature peptide coding region was cloned into Pichia pastoris for heterogeneous expression. Recombinant CFLysG showed inhibitive effect on the growth of both Gram-positive and Gram-negative bacteria with more potent activities against Gram-positive bacteria, which indicated the involvement of CFLysG in the innate immunity of C. farreri.

Developmental activation of the lysozyme gene in chicken macrophage cells is linked to core histone acetylation at its enhancer elements

Native chromatin IP assays were used to define changes in core histone acetylation at the lysozyme locus during developmental maturation of chicken macrophages and stimulation to high-level expression by lipo-polysaccharide. In pluripotent precursors the lysozyme gene (Lys) is inactive and there is no acetylation of core histones at the gene, its promoter or at the upstream cis-control elements. In myeloblasts, where there is a very low level of Lys expression, H4 acetylation appears at the cis-control elements but not at the Lys gene or its promoter: neither H3 nor H2B become significantly acetylated in myeloblasts. In mature macrophages, Lys expression increases 5-fold: H4, H2B and H2A.Z are all acetylated at the cis-control elements but H3 remains unacetylated except at the −2.4 S silencer. Stimulation with LPS increases Lys expression a further 10-fold: this is accompanied by a rise in H3 acetylation throughout the cis-control elements; H4 and H2B acetylation remain substantial but acetylation at the Lys gene and its promoter remains low. Acetylation is thus concentrated at the cis-control elements, not at the Lys gene or its immediate promoter. H4 acetylation precedes H3 acetylation during development and H3 acetylation is most directly linked to high-level Lys expression.

Gene regulation and large-scale chromatin organization in the nucleus

Regulation of gene expression involves a number of different levels of organization in the cell nucleus. The main agents of transcriptional control are the cis-acting sequences in the immediate vicinity of a gene, which combine to form the functional unit or domain. Contacts between these sequences through the formation of chromatin loops forms the most basic level of organization. The activity of functional domains is also influenced by higher order chromatin structures that impede or permit access of factors to the genes. Epigenetic modifications can maintain and propagate these active or repressive chromatin structures across large genomic regions or even entire chromosomes. There is also evidence that transcription is organized into structures called 'factories' and that this can lead to inter-chromosomal contacts between genes that have the potential to influence their regulation.

The replacement histone H2A.Z in a hyperacetylated form is a feature of active genes in the chicken

The replacement histone H2A.Z is variously reported as being linked to gene expression and preventing the spread of heterochromatin in yeast, or concentrated at heterochromatin in mammals. To resolve this apparent dichotomy, affinity-purified antibodies against the N-terminal region of H2A.Z, in both a triacetylated and non-acetylated state, are used in native chromatin immmuno-precipitation experiments with mononucleosomes from three chicken cell types. The hyperacetylated species concentrates at the 5′ end of active genes, both tissue specific and housekeeping but is absent from inactive genes, while the unacetylated form is absent from both active and inactive genes. A concentration of H2A.Z is also found at insulators under circumstances implying a link to barrier activity but not to enhancer blocking. Although acetylated H2A.Z is widespread throughout the interphase genome, at mitosis its acetylation is erased, the unmodified form remaining. Thus, although H2A.Z may operate as an epigenetic marker for active genes, its N-terminal acetylation does not.

Differentiation-dependent alterations in histone methylation and chromatin architecture at the inducible chicken lysozyme gene

It is now well established that locus-wide chromatin remodeling and dynamic alterations of histone modifications are required for the developmentally regulated activation of tissue-specific genes. However, little is known about the dynamics of these events during cell differentiation and how chromatin of an entire gene locus responds to signal transduction processes. To address this issue we investigated chromatin accessibility, linker histone distribution, and the histone methylation status at the macrophage-specific chicken lysozyme locus and the ubiquitously expressed gas41 locus in multipotent precursor cell lines and BM2 monoblast cells. The latter can be induced to go through macrophage maturation by treatment with phorbol-12-myristate acetate and can be further stimulated with bacterial lipopolysaccharide. We show that expression of the lysozyme gene in undifferentiated monoblasts is low and that a high level of gene expression requires both cell differentiation and lipopolysaccharide stimulation. However, depletion of the linker histone H1 is observed already in lysozyme non-expressing multipotent precursor cells. In undifferentiated monoblasts, the lysozyme regulatory regions are marked by the presence of monomethylated histone H3 lysine 4, which becomes increasingly converted into trimethylated H3 lysine K4 during cell differentiation. We also present evidence for extensive, differentiation-dependent alterations in nuclease accessibility at the lysozyme promoter without alterations of nucleosome and transcription factor occupancy.

The architecture of chicken chromosome territories changes during differentiation

Background

Between cell divisions the chromatin fiber of each chromosome is restricted to a subvolume of the interphase cell nucleus called chromosome territory. The internal organization of these chromosome territories is still largely unknown.

Results

We compared the large-scale chromatin structure of chromosome territories between several hematopoietic chicken cell types at various differentiation stages. Chromosome territories were labeled by fluorescence in situ hybridization in structurally preserved nuclei, recorded by confocal microscopy and evaluated visually and by quantitative image analysis. Chromosome territories in multipotent myeloid precursor cells appeared homogeneously stained and compact. The inactive lysozyme gene as well as the centromere of the lysozyme gene harboring chromosome located to the interior of the chromosome territory. In further differentiated cell types such as myeloblasts, macrophages and erythroblasts chromosome territories appeared increasingly diffuse, disaggregating to separable substructures. The lysozyme gene, which is gradually activated during the differentiation to activated macrophages, as well as the centromere were relocated increasingly to more external positions.

Conclusions

Our results reveal a cell type specific constitution of chromosome territories. The data suggest that a repositioning of chromosomal loci during differentiation may be a consequence of general changes in chromosome territory morphology, not necessarily related to transcriptional changes.

The glioma-amplified sequence 41 gene (GAS41) is a direct Myb target gene

The retroviral oncogene v-myb encodes a transcription factor (v-Myb) which transforms myelomonocytic cells in vivo and in vitro. It is thought that v-Myb exerts its biological effects by deregulating the expression of specific target genes, most of which are still unknown. The chicken glioma-amplified sequence 41 gene (GAS41) is located immediately downstream of the lysozyme gene, a known Myb-regulated gene. The GAS41 promoter colocalizes with a CpG island which also functions as an origin of replication. Since the GAS41 promoter contains several potential Myb-binding sites (MBSs) we have investigated whether GAS41 is a v-Myb target gene. Our results show that the GAS41 gene is directly activated by a v-Myb/estrogen receptor fusion protein. Furthermore, our studies reveal that the GAS41 promoter is stimulated by v-Myb in co-transfection experiments and that the DNA-binding activity of v-Myb is crucial for transactivation of the promoter. Electrophoretic mobility-shift assays (EMSA) indicate that several Myb-binding sites, residing approximately 250 bp upstream of the transcriptional start site, are bound by Myb in vitro. Furthermore, chromatin immunoprecipitation assays demonstrate that v-Myb is bound to the GAS41 promoter in vivo. Taken together these findings identify the GAS41 gene as a novel v-Myb target gene. We have also analysed the GAS41 replication origin in myelomonocytic cells and have failed to observe significant differences in origin activity in cells expressing or not expressing v-Myb.

Spatial organization of gene expression: the active chromatin hub

Developmental and tissue-specific expression of higher eukaryotic genes involves activation of transcription at the appropriate time and place and keeping it silent otherwise. Unlike housekeeping genes, tissue-specific genes generally do not cluster on the chromosomes. They can be found in gene-dense regions of chromosomes as well as in regions of repressive chromatin. Depending on the location, shielding against positive or negative regulatory effects from neighboring chromatin may be required and hence insulator and boundary models were proposed. They postulate that chromosomes are partitioned into physically distinct expression domains, each containing a gene or gene cluster with its cis-regulatory elements. Specialized elements at the borders of such domains are proposed to prevent cross-talk between domains, and thus to be crucial in establishing independent expression domains. However, genes and associated cis-acting sequences often do not occupy physically distinct domains on the chromosomes. Rather, genes can overlap and cis-acting sequences can be found tens or hundreds of kilobases away from the target gene, sometimes with unrelated genes in between. Therefore the ability of a gene to communicate with positive cis-regulatory elements rather than the presence of specialized boundary elements appears to be key to establishing an independent expression profile. Our recent finding that active beta-globin genes physically interact in the nuclear space with multiple cis-regulatory elements, with inactive genes looping out, has provided a potential mechanistic framework for this model. We refer to such a spatial unit of regulatory DNA elements as an active chromatin hub (ACH). We propose that productive ACH formation underlies correct gene expression, requiring the presence of protein factors with the appropriate affinities for each other bound to their cognate DNA sequences. Proximity and specificity determines which cis-acting sequences and promoter(s) form an ACH, and thus which gene will be expressed. Other regulatory sequences can interfere with transcription by blocking the appropriate physical interaction between an enhancer and promoter in the ACH. Possible mechanisms by which distal DNA elements encounter each other in the 3D nuclear space will be discussed.

Differential expression of the closely linked KISS1, REN, and FLJ10761 genes in transgenic mice

We previously reported the development and characterization of transgenic mice containing a large 160-kb P1 artificial chromosome (PAC) encompassing the renin (REN) locus from human chromosome 1. Here we demonstrate that PAC160 not only encodes REN, but also complete copies of the next upstream (KISS1) and downstream ( FLJ10761 ) gene along human chromosome 1. Incomplete copies of the second upstream (PEPP3) and downstream (SOX13) genes are also present. The gene order PEPP3-KISS1-REN-FLJ10761-SOX13 is conserved in mice containing either one or two copies of the REN locus. Despite the close localization of KISS1, REN, and FLJ10761 , they each exhibit distinct, yet overlapping tissue-specific expression profiles in humans. The tissue-specific expression patterns of REN and FLJ10761 were retained in transgenic mice containing PAC160. Expression of REN and FLJ10761 were also proportional to copy number. Expression of KISS1 in PAC160 mice showed both similarities and differences to humans. These data suggest that expression of gene blocks encoded on large genomic clones are retained when the clones are used to generate transgenic mice. Genomic elements which act to insulate genes from their neighbors are also apparently retained.

Dynamic reorganization of chromatin structure and selective DNA demethylation prior to stable enhancer complex formation during differentiation of primary hematopoietic cells in vitro

In order to gain insights in the true molecular mechanisms involved in cell fate decisions, it is important to study the molecular details of gene activation where such decisions occur, which is at the level of the chromatin structure of individual genes. In the study presented here we addressed this issue and examined the dynamic development of an active chromatin structure at the chicken lysozyme locus during the differentiation of primary myeloid cells from transgenic mouse bone marrow. Using in vivo footprinting we found that stable enhancer complex assembly and high-level gene expression are late events in cell differentiation. However, even before the onset of gene expression and stable transcription factor binding, specific chromatin alterations are observed. This includes changes in DNA topology and the selective demethylation of CpG dinucleotides located in the cores of critical transcription factor binding sites, but not in flanking DNA. These results firmly support the idea that epigenetic programs guiding blood cell differentiation are engraved into the chromatin of lineage-specific genes and that such chromatin changes are implemented before cell lineage specification.

Functional dissection of the mouse tyrosinase locus control region identifies a new putative boundary activity

Locus control regions (LCRs) are complex high-order chromatin structures harbouring several regulatory elements, including enhancers and boundaries. We have analysed the mouse tyrosinase LCR functions, in vitro, in cell lines and, in vivo, in transgenic mice and flies. The LCR-core (2.1 kb), located at -15 kb and carrying a previously described tissue-specific DNase I hypersensitive site, operates as a transcriptional enhancer that efficiently transactivates heterologous promoters in a cell-specific orientation-independent manner. Furthermore, we have investigated the boundary activity of these sequences in transgenic animals and cells. In mice, the LCR fragment (3.7 kb) rescued a weakly expressed reference construct that displays position effects. In Drosophila, the LCR fragment and its core insulated the expression of a white minigene reporter construct from chromosomal position effects. In cells, sequences located 5' from the LCR-core displayed putative boundary activities. We have obtained genomic sequences surrounding the LCR fragment and found a LINE1 repeated element at 5'. In B16 melanoma and L929 fibroblast mouse cells, this element was found heavily methylated, supporting the existence of putative boundary elements that could prevent the spreading of condensed chromatin from the LINE1 sequences into the LCR fragment, experimentally shown to be in an open chromatin structure.

Histone H3 lysine 4 methylation patterns in higher eukaryotic genes

Lysine residues within histones can be mono-, di - or tri-methylated. In Saccharomyces cerevisiae tri-methylation of Lys 4 of histone H3 (K4/H3) correlates with transcriptional activity, but little is known about this methylation state in higher eukaryotes. Here, we examine the K4/H3 methylation pattern at the promoter and transcribed region of metazoan genes. We analysed chicken genes that are developmentally regulated, constitutively active or inactive. We found that the pattern of K4/H3 methylation shows similarities to S. cerevisiae. Tri-methyl K4/H3 peaks in the 5' transcribed region and active genes can be discriminated by high levels of tri-methyl K4/H3 compared with inactive genes. However, our results also identify clear differences compared to yeast, as significant levels of K4/H3 methylation are present on inactive genes within the beta-globin locus, implicating this modification in maintaining a 'poised' chromatin state. In addition, K4/H3 di-methylation is not genome-wide and di-methylation is not uniformly distributed throughout the transcribed region. These results indicate that in metazoa, di- and tri-methylation of K4/H3 is linked to active transcription and that significant differences exist in the genome-wide methylation pattern as compared with S. cerevisiae.

Acetylation of histone H2B mirrors that of H4 and H3 at the chicken beta-globin locus but not at housekeeping genes

Acetylation of histones H4 and H3 targeted to promoters/enhancers is linked to the activation of transcription, whereas widespread, long range acetylation of the same histones has been linked to the requirement for open chromatin at transcriptionally active loci and regions of V(D)J recombination. Using affinity-purified polyclonal antibodies to tetra/tri-acetylated histone H2B in chromatin immunoprecipitation (ChIP) assays with mononucleosomes from 15-day chicken embryo erythrocytes, a high resolution distribution of H2B acetylation has been determined and compared with that of H4 and H3 at the same genes/loci. At the beta-globin locus, the H2B acetylation is high throughout and in general mirrors that of H3 and H4, consistent with the observation of co-precipitation of hyperacetylated H4 together with the hyperacetylated H2B. In contrast, at the weakly expressed genes glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and Gas41 (housekeeping) and carbonic anhydrase (tissue specific), very little or no hyperacetylated H2B was found despite the presence of acetylated H4 and H3 at their promoters and proximal transcribed sequences. At the inactive lysozyme and ovalbumin genes essentially no acetylation of H2B, H3, or H4 was observed. Acetylation of H2B appears to be principally a feature of only the most actively transcribed genes/loci.

Pig whey acidic protein gene is surrounded by two ubiquitously expressed genes

A 140-kb pig DNA fragment containing the whey acidic protein (WAP) gene cloned in a bacterial artificial chromosome (BAC344H5) has been shown to contain all of the cis-elements necessary for position-independent, copy-dependent and tissue-specific expression in transgenic mice. The insert from this BAC was sequenced. This revealed the presence of two other genes with quite different expression patterns in pig tissues and in transfected HC11 mouse mammary cells. The RAMP3 gene is located 15 kb upstream of the WAP gene in reverse orientation. The CPR2 gene is located 5 kb downstream of the WAP gene in the same orientation. The same locus organization was found in the human genome. The region between RAMP3 and CPR2 in the human genome contains a WAP gene-like sequence with several points of mutation which may account for the absence of WAP from human milk.

A Functional chromatin domain does not resist X chromosome inactivation: silencing of cLys correlates with methylation of a dual promoter-replication origin

To investigate the molecular mechanism(s) involved in the propagation and maintenance of X chromosome inactivation (XCI), the 21.4-kb chicken lysozyme (cLys) chromatin domain was inserted into the Hprt locus on the mouse X chromosome. The inserted fragment includes flanking matrix attachment regions (MARs), an origin of bidirectional replication (OBR), and all the cis-regulatory elements required for correct tissue-specific expression of cLys. It also contains a recently identified and widely expressed second gene, cGas41. The cLys domain is known to function as an autonomous unit resistant to chromosomal position effects, as evidenced by numerous transgenic mouse lines showing copy-number-dependent and development-specific expression of cLys in the myeloid lineage. We asked the questions whether this functional chromatin domain was resistant to XCI and whether the X inactivation signal could spread across an extended region of avian DNA. A generally useful method was devised to generate pure populations of macrophages with the transgene either on the active (Xa) or the inactive (Xi) chromosome. We found that (i) cLys and cGas41 are expressed normally from the Xa; (ii) the cLys chromatin domain, even when bracketed by MARs, is not resistant to XCI; (iii) transcription factors are excluded from lysozyme enhancers on the Xi; and (iv) inactivation correlates with methylation of a CpG island that is both an OBR and a promoter of the cGas41 gene.