Alphoid satellite DNA is tightly associated with centromere antigens in human chromosomes throughout the cell cycle

Visualization of the Genomic Loci That Are Bound by Specific Multiprotein Complexes by Bimolecular Fluorescence Complementation Analysis on Drosophila Polytene Chromosomes

We have developed a procedure that enables visualization of the genomic loci that are bound by complexes formed by a specific combination of chromatin-binding proteins. This procedure is based on imaging bimolecular fluorescence complementation (BiFC) complexes on Drosophila polytene chromosomes. BiFC complexes are formed by the facilitated association of two fluorescent protein fragments that are fused to proteins that interact with, or are in close proximity to, each other. The intensity of BiFC complex fluorescence at individual genomic loci is greatly enhanced by the parallel alignment of hundreds of chromatids within the polytene chromosomes. The loci that are bound by the complexes are mapped by comparing the locations of BiFC complex fluorescence with the stereotypical banding patterns of the chromosomes. We describe strategies for the design, expression, and validation of fusion proteins for the analysis of BiFC complex binding on polytene chromosomes. We detail protocols for the preparation of polytene chromosome spreads that have been optimized for the purpose of BiFC complex visualization. Finally, we provide guidance for the interpretation of results from studies of BiFC complex binding on polytene chromosomes and for comparison of the genomic loci that are bound by BiFC complexes with those that are bound by subunits of the corresponding endogenous complexes. The visualization of BiFC complex binding on polytene chromosomes provides a unique method to visualize multiprotein complex binding at specific loci, throughout the genome, in individual cells.

Relating centromeric topography in fixed human chromosomes to α-satellite DNA and CENP-B distribution

Despite extensive analyses on the centromere and its associated proteins, detailed studies of centromeric DNA structure have provided limited information about its topography in condensed chromatin. We have developed a method with correlative fluorescence light microscopy and atomic force microscopy that investigates the physical and structural organization of α-satellite DNA sequences in the context of its associated protein, CENP-B, on human metaphase chromosome topography. Comparison of centromeric DNA and protein distribution patterns in fixed homologous chromosomes indicates that CENP-B and α-satellite DNA are distributed distinctly from one another and relative to observed centromeric ridge topography. Our approach facilitates correlated studies of multiple chromatin components comprising higher-order structures of human metaphase chromosomes.

Repair of Ionizing Radiation Damage in Primate αDNA Transfected into Rat Cells

The time-course of repair of irradiated primate alpha DNA was studied after transfection and recovery from rat NRK cells. Rat cells were chosen for transfection because they have no alpha DNA. Plasmid pBUC4 alpha 10, containing 10 tandem 172 bp alpha DNA subunits in its 5 kbp DNA, was irradiated and introduced into the rat cells by electroporation. The transfected alpha DNA was then recovered from NRK nuclei free of extraneous rat DNA, permitting study of the fate of the transfected alpha DNA in time-course experiments. alpha DNA continuously entered nuclei for processing in the first 2.5 h after transfection. The pool of damaged bases in alpha DNA in NRK nuclei was detectable 2.5 h after transfection. Radiation-induced alpha DNA fragments of electrophoretic mobility intermediate between those of unit nucleotide length were prominent in sequencing gel analyses of alpha DNA for 5-150 min after transfection. These intermediate mobility fragments initially disappeared with T 1/2 of 6-20 min. The alpha DNAs of intermediate mobility are presumed to be intermediates in DNA repair. Residual DNA base damage which had not been processed in the transfected cells could later be unmasked in vitro by conversion to strand breaks by beta-elimination using heat and piperidine or endonuclease III of E. coli. Irradiation of the recipient NRK cells with 5 Gy 4 hours before transfection prolonged the time during which intermediate mobility species could be found, consistent with the increased frequency of intermediate mobility species observed in DNA of monkey CV-1 cells pretreated with small doses of radiation before 300 Gy (Bases et al. 1990).

A minimal CENP-A core is required for nucleation and maintenance of a functional human centromere

Chromatin clusters containing CENP-A, a histone H3 variant, are found in centromeres of multicellular eukaryotes. This study examines the ability of alpha-satellite (alphoid) DNA arrays in different lengths to nucleate CENP-A chromatin and form functional kinetochores de novo. Kinetochore assembly was followed by measuring human artificial chromosome formation in cultured human cells and by chromatin immunoprecipitation analysis. The results showed that both the length of alphoid DNA arrays and the density of CENP-B boxes had a strong impact on nucleation, spreading and/or maintenance of CENP-A chromatin, and formation of functional kinetochores. These effects are attributed to a change in the dynamic balance between assembly of chromatin containing trimethyl histone H3-K9 and chromatin containing CENP-A/C. The data presented here suggest that a functional minimum core stably maintained on 30-70 kb alphoid DNA arrays represents an epigenetic memory of centromeric chromatin.

Assembly of additional heterochromatin distinct from centromere-kinetochore chromatin is required for de novo formation of human artificial chromosome

Alpha-satellite (alphoid) DNA is necessary for de novo formation of human artificial chromosomes (HACs) in human cultured cells. To investigate the relationship among centromeric, transcriptionally permissive and non-permissive chromatin assemblies on de novo HAC formation, we constructed bacterial artificial chromosome (BAC)-based linear HAC vectors whose left vector arms are occupied by beta geo coding genes with or without a functional promoter in addition to a common marker gene on the right arm. Although HACs were successfully generated from the vectors with promoter-less constructs on the left arm in HT1080 cells, we failed to generate a stable HAC from the vectors with a functional promoter on the left arm. Despite this failure in HAC formation, centromere components (CENP-A, CENP-B and CENP-C) assembled at the integration sites correlating with a transcriptionally active state of both marker genes on the vector arms. However, on the stable HAC, chromatin immunoprecipitation analysis showed that HP1alpha and trimethyl histone H3-K9 were enriched at the non-transcribing left vector arm. A transcriptionally active state on both vector arms is not compatible with heterochromatin formation on the introduced BAC DNA, suggesting that epigenetic assembly of heterochromatin is distinct from centromere chromatin assembly and is required for the establishment of a stable artificial chromosome.

An artificially constructed de novo human chromosome behaves almost identically to its natural counterpart during metaphase and anaphase in living cells

Human artificial chromosomes (HACs) are promising reagents for the analysis of chromosome function. While HACs are maintained stably, the segregation mechanisms of HACs have not been investigated in detail. To analyze HACs in living cells, we integrated 256 copies of the Lac operator into a precursor yeast artificial chromosome (YAC) containing alpha-satellite DNA and generated green fluorescent protein (GFP)-tagged HACs in HT1080 cells expressing a GFP-Lac repressor fusion protein. Time-lapse analyses of GFP-HACs and host centromeres in living mitotic cells indicated that the HAC was properly aligned at the spindle midzone and that sister chromatids of the HAC separated with the same timing as host chromosomes and moved to the spindle poles with mobility similar to that of the host centromeres. These results indicate that a HAC composed of a multimer of input alpha-satellite YACs retains most of the functions of the centromeres on natural chromosomes. The only difference between the HAC and the host chromosome was that the HAC oscillated more frequently, at higher velocity, across the spindle midzone during metaphase. However, this provides important evidence that an individual HAC has the capacity to maintain tensional balance in the pole-to-pole direction, thereby stabilizing its position around the spindle midzone.

Combined multicolor-FISH and immunostaining

The combination of multicolor-FISH and immunostaining produces a powerful visual method to analyze in situ DNA-protein interactions and dynamics. Representing one of the major technical improvements of FISH technology, this method has been used extensively in the field of chromosome and genome research, as well as in clinical studies, and serves as an important tool to bridge molecular analysis and cytological description. In this short review, the development and significance of this method will be briefly summarized using a limited number of examples to illustrate the large body of literature. In addition to descriptions of technical considerations, future applications and perspectives have also been discussed focusing specifically on the areas of genome organization, gene expression and medical research. We anticipate that this versatile method will play an important role in the study of the structure and function of the dynamic genome and for the development of potential applications for medical research.

Structural and Functional Dynamics of Human Centromeric Chromatin

Centromeres are the elements of chromosomes that assemble the proteinaceous kinetochore, maintain sister chromatid cohesion, regulate chromosome attachment to the spindle, and direct chromosome movement during cell division. Although the functions of centromeres and the proteins that contribute to their complex structure and function are conserved in eukaryotes, centromeric DNA diverges rapidly. Human centromeres are particularly complicated. Here, we review studies on the organization of homogeneous arrays of chromosome-specific alpha-satellite repeats and evolutionary links among eukaryotic centromeric sequences. We also discuss epigenetic mechanisms of centromere identity that confer structural and functional features of the centromere through DNA-protein interactions and post-translational modifications, producing centromere-specific chromatin signatures. The assembly and organization of human centromeres, the contributions of satellite DNA to centromere identity and diversity, and the mechanism whereby centromeres are distinguished from the rest of the genome reflect ongoing puzzles in chromosome biology.

Cell-type specific proximity of centromeric domains of one homologue each of chromosomes 2 and 11 in nuclei of cerebellar Purkinje neurons

In Purkinje neurons of the mouse cerebellum, the centromeres of several chromosomes are placed in close proximity to form a distinct pattern of clusters and exhibit reproducible spatial redistributions during development. In granule neurons, an adjacent cell type in the cerebellum, the pattern, size, and number of centromeric aggregations are different from those of Purkinje neurons. The present work was undertaken to test the hypothesis that the same chromosomes form part of one aggregate in a cell-type-specific manner. Fluorescence in situ hybridization (FISH) with chromosome-specific paracentromeric probes was used to identify centromeric regions of individual chromosomes in cerebellar Purkinje and granule neurons of the adult mouse. When pairs of centromeric probes were used in two-color FISH, one homologue each of chromosomes 2 and 11 were routinely found close to each other in Purkinje neurons but not in granule neurons. This finding of specific proximity was limited to the pair 2 and 11, out of the ten chromosome pairs that were randomly selected and studied. Our results indicate that, in adult Purkinje neurons, a cell-type-specific spatial proximity is present between centromeric domains of one homologue each of chromosomes 2 and 11.

Ikaros-family proteins: in search of molecular functions during lymphocyte development

The regulatory steps that lead to the differentiation of hematopoietic cells from a multipotential stem cell remain largely unknown. A beginning to the understanding of these steps has come from the study of DNA-binding proteins that are thought to regulate the expression of genes required for specific developmental events. Ikaros is the founding member of a small family of DNA-binding proteins required for lymphocyte development, but the members of this family differ from other key regulators of lymphopoiesis in that direct target genes have not been conclusively identified, and reasonable support has been presented for only a few potential targets. Therefore, the molecular mechanisms that Ikaros uses for regulating lymphocyte development remain largely unknown. Current data suggest that, in some instances, Ikaros may function as a typical transcription factor. However, recent results suggest that it may function more broadly, perhaps in the formation of silent and active chromatin structures. In this review, our current knowledge of the molecular functions of Ikaros will be discussed.

Rapid generation of long synthetic tandem repeats and its application for analysis in human artificial chromosome formation

Human artificial chromosomes (HACs) provide a unique opportunity to study kinetochore formation and to develop a new generation of vectors with potential in gene therapy. An investigation into the structural and the functional relationship in centromeric tandem repeats in HACs requires the ability to manipulate repeat substructure efficiently. We describe here a new method to rapidly amplify human alphoid tandem repeats of a few hundred base pairs into long DNA arrays up to 120 kb. The method includes rolling-circle amplification (RCA) of repeats in vitro and assembly of the RCA products by in vivo recombination in yeast. The synthetic arrays are competent in HAC formation when transformed into human cells. As short multimers can be easily modified before amplification, this new technique can identify repeat monomer regions critical for kinetochore seeding. The method may have more general application in elucidating the role of other tandem repeats in chromosome organization and dynamics.

Characterization of a CENP-C homolog in Arabidopsis thaliana

Centromere protein C (CENP-C) is a component of the kinetochore essential for correct segregation of sister chromatids in mammals. In Arabidopsis thaliana, a single-copy gene encoding a protein homologous to CENP-C has been found by homology in the whole-genome sequence. To investigate the CENP-C homolog (AtCENP-C), we cloned cDNAs by RT-PCR and determined its full-length coding sequence. Antibodies against the synthetic peptide for the C-terminal residues of AtCENP-C detected a polypeptide in Arabidopsis cell extracts on western blots. Immunofluorescence labeling with the antibodies and fluorescence in situ hybridization demonstrated clearly that AtCENP-C is present at the centromeric regions throughout the cell cycle.

Construction of a novel human artificial chromosome vector for gene delivery

Potential problems of conventional transgenes include insertional disruption of the host genome and unpredictable, irreproducible expression of the transgene by random integration. Alternatively, human artificial chromosomes (HACs) can circumvent some of the problems. Although several HACs were generated and their mitotic stability was assessed, a practical way for introducing exogenous genes by the HACs has yet to be explored. In this study, we developed a novel HAC from sequence-ready human chromosome 21 by telomere-directed chromosome truncation and added a loxP sequence for site-specific insertion of circular DNA by the Cre/loxP system. This 21HAC vector, delivered to a human cell line HT1080 by microcell fusion, bound centromere proteins A, B, and C and was mitotically stable during long-term culture without selection. The EGFP gene inserted in the HAC vector expressed persistently. These results suggest that the HAC vector provides useful system for functional studies of genes in isogenic cell lines.

CENP-B Interacts with CENP-C Domains Containing Mif2 Regions Responsible for Centromere Localization

Recently, human artificial chromosomes featuring functional centromeres have been generated efficiently from naked synthetic alphoid DNA containing CENP-B boxes as a de novo mechanism in a human cultured cell line, but not from the synthetic alphoid DNA only containing mutations within CENP-B boxes, indicating that CENP-B has some functions in assembling centromere/kinetochore components on alphoid DNA. To investigate whether any interactions exist between CENP-B and the other centromere proteins, we screened a cDNA library by yeast two-hybrid analysis. An interaction between CENP-B and CENP-C was detected, and the CENP-C domains required were determined to overlap with three Mif2 homologous regions, which were also revealed to be involved in the CENP-C assembly of centromeres by expression of truncated polypeptides in cultured cells. Overproduction of truncated CENP-B containing no CENP-C interaction domains caused abnormal duplication of CENP-C domains at G2 and cell cycle delay at metaphase. These results suggest that the interaction between CENP-B and CENP-C may be involved in the correct assembly of CENP-C on alphoid DNA. In other words, a possible molecular linkage may exist between one of the kinetochore components and human centromere DNA through CENP-B/CENP-B box interaction.

Epigenetic assembly of centromeric chromatin at ectopic alpha-satellite sites on human chromosomes

To investigate the mechanism of chromatin assembly at human centromeres, we isolated cultured human cell lines in which a transfected alpha-satellite (alphoid) YAC was integrated ectopically into the terminal region of host chromosome 16, where it was stably maintained. Centromere activity of the alphoid YAC was suppressed at ectopic locations on the host chromosome, as indicated by the absent or reduced assembly of CENP-A and -C. However, long-term culture in selective medium, or short-term treatment with the histone deacetylase inhibitor Trichostatin A (TSA), promoted the re-assembly of CENPA, -B and -C at the YAC site and the release of minichromosomes containing the YAC integration site. Chromatin immunoprecipitation analyses of the re-formed minichromosome and the alphoid YAC-based stable human artificial chromosome both indicated that CENP-A and CENP-B assembled only on the inserted alphoid array but not on the YAC arms. On the YAC arms at the alphoid YAC integration sites, TSA treatment increased both the acetylation level of histone H3 and the transcriptional level of a marker gene. An increase in the level of transcription was also observed after long-term culture in selective medium. These activities, which are associated with changes in chromatin structure, might reverse the suppressed chromatin state of the YAC at ectopic loci, and thus might be involved in the epigenetic change of silent centromeres on ectopic alphoid loci.

Coordinated and conserved expression of alphoid repeat and alphoid repeat-tagged coding sequences

We have found an alpha-like simple-sequence DNA repeat that is differentially expressed during early embryogenesis in both chick and zebrafish. Before and during the primitive streak stage, transcripts of the alphoid repeat sequence were ubiquitously expressed throughout zebrafish and chick embryos. After headfold formation, expression was limited to the cardiac neural crest, the head, and the heart. Two types of alphoid repeat sequence transcripts were identified: alphoid repeat RNA and alphoid repeat-tagged mRNA (ESalphaT). Several of the ESalphaTs were identified by (1) searching expressed sequence tag databases, (2) arbitrary rapid amplification of cDNA ends (RACE), and (3) screening embryonic cDNA libraries. The alphoid element was located in the 3' untranslated region of one ESalphaT that was obtained by RACE. The ESalphaT sequences encoded a variety of different types of proteins, but all were expressed within tissues that were positive for the alphoid repeat RNA. The presence of two types of coordinately expressed alphoid-like repeat transcripts in maternal RNA with subsequent restriction to the head and heart, and the conservation of these features in disparate vertebrate embryos, suggest that the alphoid repeat sequence may serve as a control element in the gene regulation network.

Polyploid cells in the mouse ovary

Cell ploidy in the ovarian follicle and corpus luteum was investigated by DNA in situ hybridization to a reiterated, chromosome 3 transgene in mice that were hemizygous for the transgene. This approach was first validated by analysis of mouse kidney, pancreas and liver control tissues, which contain different frequencies of polyploid nuclei. Polyploid nuclei (with multiple hybridization signals) were seen in histological sections of both ovarian follicles and corpora lutea. The frequency of polyploid nuclei in follicles showed no consistent relationship with age (between 6 weeks and 10 months) but polyploid nuclei were significantly more abundant in corpora lutea than follicles (6.3% vs. 2.5%). This implies that production of polyploid cells is more closely associated with differentiation of ovarian follicles into corpora lutea than with the age of the female. Polyploidy tended to be more frequent in corpora lutea of mice that had mated even if they did not become pregnant. This study has highlighted the presence of polyploid cells in the mouse ovarian follicle and corpus luteum and has identified mating as a possible trigger for polyploidy in the corpus luteum. Further work is required to determine the physiological role of polyploid ovarian cells in reproduction.

Beyond the ABCs of CKC and SCC. Do centromeres orchestrate sister chromatid cohesion or vice versa?

The centromere-kinetochore complex is a highly specialized chromatin domain that both mediates and monitors chromosome-spindle interactions responsible for accurate partitioning of sister chromatids to daughter cells. Centromeres are distinguished from adjacent chromatin by specific patterns of histone modification and the presence of a centromere-specific histone H3 variant (e.g. CENP-A). Centromere-proximal regions usually correspond to sites of avid and persistent sister chromatid cohesion mediated by the conserved cohesin complex. In budding yeast, there is a substantial body of evidence indicating centromeres direct formation and/or stabilization of centromere-proximal cohesion. In other organisms, the dependency of cohesion on centromere function is not as clear. Indeed, it appears that pericentromeric heterochromatin recruits cohesion proteins independent of centromere function. Nonetheless, aspects of centromere function are impaired in the absence of sister chromatid cohesion, suggesting the two are interdependent. Here we review the nature of centromeric chromatin, the dynamics and regulation of sister chromatid cohesion, and the relationship between the two.

Molecular behavior in living mitotic cells of human centromere heterochromatin protein HPLalpha ectopically expressed as a fusion to red fluorescent protein

We constructed stable mammalian cell lines in which human heterochromatin protein HP1alpha and kinetochore protein CENP-A were differentially expressed as fusions to red (RFP-HP1) and green fluorescent proteins (GFP-CENP-A). Heterochromatin localization of RFP-HP1 was clearly shown in mouse and Indian muntjac cells. By preparing mitotic chromosome spreads, the inner centromere localization of RFP-HP1 was observed in human and Indian muntjac cells. To characterize its molecular behavior in living mitotic cells, time-lapse images of RFP-HP1 were obtained by computer-assisted image analyzing system, mainly with mouse cells. In G2 phase, a significant portion of RFP-HP1 diffused homogeneously in the nucleus and further dispersed into the cytoplasm soon after the nuclear membrane breakdown, while some remained in the centromeric region. Simultaneous observations with GFP-CENP-A in human cells showed that RFP-HP1 was located just between the sister kinetochores and then aligned to the spindle midzone. With the onset of anaphase, once it was released from there, it moved to the centromeres of segregating chromosomes or returned to the spindle equator. As cytokinesis proceeded, HP1alpha was predominantly found in the newly formed daughter nuclei and again displayed a heterochromatin-like distribution. These results suggested that, although the majority of HP1alpha diffuses into the cytoplasm, some populations are retained in the centromeric region and involved in the association and segregation of sister kinetochores during mitosis.

Degradation of nucleosome-associated centromeric histone H3-like protein CENP-A induced by herpes simplex virus type 1 protein ICP0

Cells infected by herpes simplex virus type 1 in the G2 phase of the cell cycle become stalled at an unusual stage of mitosis defined as pseudoprometaphase. This block correlates with the viral immediate-early protein ICP0-induced degradation of the centromere protein CENP-C. However, the observed pseudoprometaphase phenotype of infected mitotic cells suggests that the stability of other centromere proteins may also be affected. Here, we demonstrate that ICP0 also induces the proteasome-dependent degradation of the centromere protein CENP-A. By a series of Western blot and immunofluorescence experiments we show that the endogenous 17-kDa CENP-A and an exogenous tagged version of CENP-A are lost from centromeres and degraded in infected and transfected cells as a result of ICP0 expression. CENP-A is a histone H3-like protein associated with nucleosome structures in the inner plate of the kinetochore. Unlike fully transcribed lytic viral DNA, the transcriptionally repressed latent herpes simplex virus type 1 genome has been reported to have a nucleosomal structure similar to that of cellular chromatin. Because ICP0 plays an essential part in controlling the balance between the lytic and latent outcomes of infection, the ICP0-induced degradation of CENP-A is an intriguing feature connecting different aspects of viral and/or cellular genome regulation.

Transcription modulation by a rat nuclear scaffold protein, P130, and a rat highly repetitive DNA component or various types of animal and plant matrix or scaffold attachment regions

The XmnI fragment, a highly repetitive DNA component, and animal and plant matrix or scaffold attachment region (MAR/SAR) were examined for similarity in interaction with nuclear scaffold. As the XmnI fragment bound a 130 kDa scaffold protein (P130) in vitro, various types of MAR/SAR fragments could bind 130 and 123 kDa scaffold proteins. The native XmnI and MAR/SAR fragments clearly augmented SV40 promoter-mediated luciferase gene transcription following transient transfection of recombinant plasmids into various types of recipient cells. In contrast, the XmnI fragment methylated at the cytosine base of the unique HindIII site, and a synthetic variant DNA deficient in base unpairing characteristic of MAR/SAR, could neither bind P130 nor augment this transcription. These two types of genomic regions appeared to have similar properties of interaction with nuclear scaffold, by which the activity of appropriately positioned promoter can be modulated.

Genomic organization, chromosomal localization, and the complete 22 kb DNA sequence of the human GCMa/GCM1, a placenta-specific transcription factor gene

The genomic sequence of the human GCMa/GCM1 gene, a mammalian homologue of Drosophila melanogaster GCM, was determined. Drosophila GCM is a neural transcription factor that regulates glial cell fate. The mammalian homolog however, is a placenta-specific transcription factor that is necessary for placental development. The 22 kb DNA sequence spanning the GCMa gene contains six exons and five introns, encoding a 2.8 kb cDNA. Overall genomic organization is similar for the human and mouse. Several potential binding sites for transcription factors like GATA, Oct-1, and bHLH proteins were found in the 5'-flanking region of the human gene. A DNA motif for GCM protein binding exists in the 5'-flanking region that is highly homologous with that of the mouse gene. The location of this gene was mapped to chromosome 6 using fluorescence in situ hybridization.

A satellite DNA containing CENP-B box-like motifs is present in the antarctic scallop Adamussium colbecki

The DNA of the Antarctic scallop Adamussium colbecki was found to contain a highly repeated sequence identifiable upon restriction with endonuclease BglII. The monomeric unit - denominated pACS (about 170bp long) - was cloned. Southern blot hybridization yielded a ladder-like banding pattern, indicating that the repeated elements are tandemly arranged in the genome and therefore represent a sequence of satellite DNA. Sequence analysis of five different clones revealed the presence of various subfamilies, some of which showed a high degree of divergence. In each clone, regions homologous to the mammalian CENP-B box were observed. A region homologous to the CDEIII centromeric sequence of yeast was also found in one of the clones. These observations suggest a relationship of the pACS family to the centromeric area in A. colbecki.

Nuclear topology of murine, cerebellar Purkinje neurons: changes as a function of development

The interphase nucleus is a structurally ordered, three-dimensional structure, in which specific chromatin domains occupy distinct spatial positions that can, in turn, be modified with changes in cell function. A fundamental goal in developmental neurobiology is the identification of mechanisms that dictate the orderly expression of genes in a cell-specific manner. Given that different neuronal populations feature a characteristic spatial topology of centromeric sequences, the positioning of specific DNA sequences may constitute such a mechanism. We tested the hypothesis that the cell-specific nuclear topology in fully differentiated neurons is acquired before or during that stage at which neuron-specific sequences are first expressed. For this, we assessed the number and spatial distribution of centromeric domains in the murine, cerebellar Purkinje neuron as a function of postnatal development. Centromeric domains were localized by immunofluorescence of centromere-associated kinetochore proteins and visualized by confocal microscopy. Kinetochores are known to cluster in Purkinje neurons. Thus, the number of signals discerned is always less than the chromosome complement of the species. The number of signals observed in adults (10.8 +/- 0.46) (mean +/- SEM) is established by postnatal day 15 (P15), after a transient decrease from 11.44 +/- 0.44 at P0 to 8.78 +/- 0.24 at P3. The distribution of signals characteristic of the adult, with the majority located at the nucleolus, is established by P5 and is associated with a decrease in the fraction of signals at the nuclear periphery. These changes are temporally associated with the onset of processes such as dendritic differentiation and synaptic maturation and might serve the process of differentiation by placing specific sequences into transcriptionally competent, nuclear sites.

Association of DNAse sensitive chromatin domains with the nuclear periphery in 3T3 cells in vitro

DNAse sensitive chromatin, putative transcriptionally competent sequences, exists either as pan-nuclear speckles in cells with nuclei which exhibit a flat geometry, or as a shell apposed to the nuclear envelope in cells with spheroidal nuclei. To test the hypothesis that DNAse sensitive chromatin is similarly associated with the nuclear periphery in cell types with a very flat geometry such as 3T3 fibroblasts, cells were subjected to hypotonic expansion to change their nuclei from a flat ellipsoid to a spheriod. This was based on the assumption that such a spatial association is not resolvable due to the interdigitation at the nuclear midplane of DNAse sensitive chromatin associated with the upper and lower nuclear surfaces. In situ nick translation was used to visualize the distribution of DNAse sensitive chromatin as a function of nuclear geometry. Both unexpanded and expanded cells exhibit DNAse sensitive chromatin as a dome at the apical side of the nucleus, i.e., that aspect of the cell facing the culture medium. The results argue for a polarized association of DNAse sensitive chromatin with the nuclear envelope and indicate that the nuclear periphery may function as a compartment for the spatial coupling of transcription and nucleo-cytoplasmic transport.

Key words: nuclear organization, DNAse sensitive chromatin, hypotonic expansion, 3T3 cells.

Visualization of prekinetochore locus on the centromeric region of highly extended chromatin fibers: does kinetochore autoantigen CENP-C constitute a kinetochore organizing center?

Kinetochore is morphologically defined as a trilaminated, highly differentiated structure at the primary constriction of mitotic chromosomes. This subcellular organella is assumed to be composed of DNA and proteins. Immunoelectron microscopy has shown that centromere autoantigens CENP-C and CENP-B localize to the kinetochore inner plate and the underlying centromeric region respectively. We previously indicated that both are DNA-binding proteins that constitute centromeric heterochromatin throughout the cell cycle. Here, we tried to elucidate how these molecules are involved in the kinetochore/centromere organization in vivo by analyzing their morphological behavior in nuclei. Using immunofluorescence microscopy, we found that CENP-C remained as round discrete dots, whereas CENP-B displayed larger surrounding materials. To examine the CENP-C-binding locus on the genome, we prepared highly extended chromatin fibers and performed simultaneous immunofluorescence and fluorescence in situ hybridization. We obsreved that centromeric alphoid DNA, targeted by CENP-B, was highly dispersed, whereas the CENP-C antigen persisted as small dots well situated on the fibers. These features reminded us of the 'ball and cup' structure that had been presented for 'prekinetochore'. We propose here that CENP-C constitutes a 'kinetochore organizing center' tightly associating with DNA, whereas CENP-B heterochromatin offers the solid support during kinetochore maturation.

Structural requirements and dynamics of mitosin-kinetochore interaction in M phase

Mitosin is a 350-kDa human nuclear protein which transiently associates with centromeres and spindle poles in M phase. Ultrastructure studies reveal that it is located at the outer kinetochore plate. In this work, we explored the detailed structural basis and dynamics of the mitosin-kinetochore interaction. Two major regions important for targeting to centromeres were identified by analyzing different deletion mutants expressed in CHO cells: (i) the "core region" between amino acids 2792 and 2887, which was essential for the centromere localization of mitosin; and (ii) the internal repeats between residues 2094 and 2487, which cooperated with the core region to achieve strong mitosin-kinetochore interaction. The core region is characteristic of two leucine zipper motifs. Deletion of either motif abolished the centromere localization activity. In addition, Cys2864, adjacent to the second motif, was also essential for the activity of the core region. In contrast, the internal repeats alone were insufficient for centromere localization. We propose that this region may serve as a regulatory domain to facilitate interaction of the core region with the kinetochore. We showed that mitosin molecules entering nuclei after nuclear envelope breakdown (NEBD) were not assembled onto kinetochores efficiently, suggesting that the mitosin-kinetochore interaction is stabilized prior to NEBD. This result supports the idea of an ordered process for kinetochore assembly. Our data also suggest that mitosin might interact with chromatin in interphase. Evidence for coordinated regulation between the centromere-targeting and the putative chromatin-binding activities is also provided.

Clinical and immunological characterization of patients with systemic sclerosis overlapping primary biliary cirrhosis: a comparison with patients with systemic sclerosis alone

Limited cutaneous systemic sclerosis (lSSc) or CREST syndrome is sometimes complicated by primary biliary cirrhosis (PBC). To characterize the clinical and immunological features of patients with SSc overlapping PBC (SSc-PBC), the clinical and laboratory data of 11 SSc-PBC were studied. Since all of the SSc-PBC were lSSc, the features of SSc-PBC were compared with those of 266 patients with lSSc alone. The sera from SSc-PBC showed a significantly higher positivity for anti-centromere antibody (ACA) (p < 0.0005) and higher reactivity to centromere protein-C. Although SSc-PBC more frequently exhibited calcinosis and telangiectasia (p < 0.05), there was no statistical difference in other features of CREST syndrome. There was no statistical difference in other scleroderma-related features either. SSc-PBC were more frequently complicated with Sjögren syndrome (p < 0.05). The most prominent feature of SSc-PBC was a higher incidence of ACA than in lSSc alone.

Involvement of DNA methylation in binding of a highly repetitive DNA component to nuclear scaffold proteins from rat liver

Experimental reduction of the amount of CpG methylation in a highly repetitive DNA component was achieved by growth of Ac2F cells in the presence of 5-aza-2'-deoxycytidine or procainamide, as judged by the results of methyl-sensitive restriction endonuclease digestion and colony hybridization. Modification of genomic DNA with these DNA methylation inhibitors increased the release of 370-bp highly repetitive DNA from rat chromosomal DNA by HindIII digestion. This result indicated that highly repetitive DNA components in the nuclear scaffold fraction are hypermethylated. On the other hand, methylated DNA was used for southwestern analysis to investigate the protein(s) which bind specifically to the DNA in the nuclear scaffold fraction. The introduction of additional methylated cytosines within a highly repetitive DNA component affected the binding of DNA to the nuclear scaffold proteins. Thus, cytosine methylation may be involved in the regulation of gene expression and construction of the higher-order structure of chromatin.

Centromeric protein B null mice are viable with no apparent abnormalities

The centromere protein B (CENP-B) is a centromeric DNA/binding protein. It recognizes a 17-bp sequence motif called the CENP-B box, which is found in the centromeric region of most chromosomes. It binds DNA through its amino terminus and dimerizes through its carboxy terminus. CENP-B protein has been proposed to perform a vital role in organizing chromatin structures at centromeres. However, other evidence does not agree with this view. For example, CENP-B is found at inactive centromeres on stable dicentric chromosomes, and also mitotically stable chromosomes lacking alpha-satellite DNA have been reported. To address the biological function of CENP-B, we generated mouse null mutants of CENP-B by homologous recombination. Mice lacking CENP-B were viable and fertile, indicating that mice without CENP-B undergo normal somatic and germline development. Thus, both mitosis and meiosis are able to proceed normally in the absence of CENP-B.

Interphase-specific association of intrinsic centromere protein CENP-C with HDaxx, a death domain-binding protein implicated in Fas-mediated cell death

CENP-C, one of the few known intrinsic proteins of the human centromere, is thought to play structural as well as regulatory roles crucial to proper chromosome segregation and mitotic progression. To further define the functions of CENP-C throughout the cell cycle we have used the yeast interaction trap to identify proteins with which it interacts. One specific CENP-C interactor, which we have named HDaxx, was characterized in detail and found to be homologous to murine Daxx, a protein identified through its ability to bind the death domain of Fas (CD95). The interaction between CENP-C and HDaxx is mediated by the amino-terminal 315 amino acids of CENP-C and the carboxyl-terminal 104 amino acids of HDaxx. This region of Daxx is responsible for binding to death domains of several apoptosis signalling proteins. The biological significance of the interaction between CENP-C and HDaxx was confirmed by immunofluorescence colocalization of these two proteins at discrete spots in the nuclei of some interphase HeLa cells. We discuss the functional implications of the interphase-restricted association of HDaxx with centromeres.

Construction of YAC-based mammalian artificial chromosomes

To construct a mammalian artificial chromosome (MAC), telomere repeats and selectable markers were introduced into a 100 kb yeast artificial chromosome (YAC) containing human centromeric DNA. This YAC, which has a regular repeat structure of alpha-satellite DNA and centromere protein B (CENP-B) boxes, efficiently formed MACs that segregated accurately and bound CENP-B, CENP-C, and CENP-E. The MACs appear to be about 1-5 Mb in size and contain YAC multimers. Structural analyses suggest that the MACs have not acquired host sequences and were formed by a de novo mechanism. The accurate segregation of the MACs suggests they have potential as vectors for introducing genes into mammals.

Centromere protein B null mice are mitotically and meiotically normal but have lower body and testis weights

CENP-B is a constitutive centromere DNA-binding protein that is conserved in a number of mammalian species and in yeast. Despite this conservation, earlier cytological and indirect experimental studies have provided conflicting evidence concerning the role of this protein in mitosis. The requirement of this protein in meiosis has also not previously been described. To resolve these uncertainties, we used targeted disruption of the Cenpb gene in mouse to study the functional significance of this protein in mitosis and meiosis. Male and female Cenpb null mice have normal body weights at birth and at weaning, but these subsequently lag behind those of the heterozygous and wild-type animals. The weight and sperm content of the testes of Cenpb null mice are also significantly decreased. Otherwise, the animals appear developmentally and reproductively normal. Cytogenetic fluorescence-activated cell sorting and histological analyses of somatic and germline tissues revealed no abnormality. These results indicate that Cenpb is not essential for mitosis or meiosis, although the observed weight reduction raises the possibility that Cenpb deficiency may subtly affect some aspects of centromere assembly and function, and result in reduced rate of cell cycle progression, efficiency of microtubule capture, and/or chromosome movement. A model for a functional redundancy of this protein is presented.

Involvement of phosphorylation in binding of nuclear scaffold proteins from rat liver to a highly repetitive DNA component

The results of our previous work [Hibino et al., Biochim. Biophys. Acta 1174 (1993) 162-170] suggested that a highly repetitive DNA component facilitates bending of the helix axis to be recognized by the nuclear scaffold proteins from rat liver, P123 and P130. In the present experiment, it was shown that binding of these proteins to such a repetitive DNA component from rat liver nuclei (370-bp XmnI fragment) is based on a cooperative mode of interaction, although the binding activity of P130 is much higher than that of P123. The immunoblot analysis with anti-phosphoamino acid antibodies suggested that phosphorylation of serine and threonine residues occurs on P123 and P130, but also of tyrosine residue(s) on P130. The phosphatase assay showed that phosphoryl groups on these proteins may be involved in altering the DNA binding activities of the proteins. Thus, the results in the present study imply that phosphorylation of a nuclear scaffold protein in addition to the degree of bending of the DNA helix axis plays an important role in anchoring chromatin to the nuclear scaffold and in construction of a higher-order chromatin structure.

The distribution of binding sites for centromere protein B (CENP-B) is partly conserved among diverged higher order repeating units of human chromosome 6-specific alphoid DNA

We previously reported the isolation of alphoid satellite clones from a human genomic library using a DNA immunoprecipitation with centromere protein B (CENP-B). Here, we have characterized the distribution of CENP-B-binding sites on the 3-kb BamHI repeats of the cos2 clone. Using in situ hybridization, this alphoid satellite was located primarily at the centromeric region of chromosome 6. The functional binding sites were mapped by precipitating the restriction fragments with recombinant CENP-B in vitro. One repeat (2B3-11) consisted of 19 copies of alphoid monomer, eight of which possessed the binding sites, while another (2B3-9) consisted of 18 copies of the monomer, seven of which possessed the binding sites. The distribution of the sites was well conserved between them, except for the terminus. A similar analysis with the remaining 6-kb region suggested the presence of a continuous 1-kb region with regular spacing of EcoRI sites and the CENP-B-binding sites. When the nucleotide sequence of 2B3-11 was compared with that of another chromosome 6-specific alphoid repeat (p308) that had been described previously, this 1-kb region was highly conserved between them. The distribution of the CENP-B binding sites and the order of alphoid monomers might define the folding of alphoid repeats in the centromeric region.

Centromere protein B of African green monkey cells: gene structure, cellular expression, and centromeric localization

Centromere protein B (CENP-B) is a centromeric DNA-binding protein which recognizes a 17-bp sequence (CENP-B box) in human and mouse centromeric satellite DNA. The African green monkey (AGM) is phylogenetically closer to humans than mice and is known to contain large amounts of alpha-satellite DNA, but there has been no report of CENP-B boxes or CENP-B in the centromere domains of its chromosomes. To elucidate the AGM CENP-B-CENP-B box interaction, we have analyzed the gene structure, expression, biochemical properties, and centromeric localization of its CENP-B. The amino acid sequence deduced from the cloned AGM CENP-B gene was established to be highly homologous to that of human and mouse CENP-B. In particular, the DNA binding and homodimer formation domains demonstrated 100% identity to their human and mouse counterparts. Immunoblotting and DNA mobility shift analyses revealed CENP-B to be expressed in AGM cell lines. As predicted from the gene structure, the AGM CENP-B in the cell extracts exhibited the same DNA binding specificity and homodimer forming activity as human CENP-B. By indirect immunofluorescent staining of AGM mitotic cells with anti-CENP-B antibodies, a centromere-specific localization of AGM CENP-B could be demonstrated. We also isolated AGM alpha-satellite DNA with a CENP-B box-like sequence with CENP-B affinity. These results not only prove that CENP-B functionally persists in AGM cells but also suggest that the AGM genome contains the recognition sequences for CENP-B (CENP-B boxes with the core recognition sequence or CENP-B box variants) in centromeric satellite DNA.

Specific interaction between human kinetochore protein CENP-C and a nucleolar transcriptional regulator

CENP-C is a human kinetochore protein that was originally identified as a chromosomal autoantigen in patients with scleroderma spectrum disease. To begin to establish a comprehensive protein map of the human centromere, affinity chromatography was used to identify nuclear proteins that specifically interact with CENP-C. Whereas a number of polypeptides appeared to interact with the full-length CENP-C protein, only a pair of similarly sized proteins of approximately 100 kDa specifically interacted with the isolated carboxyl-terminal third of the CENP-C protein. Neither protein of the doublet bound to control affinity columns. Affinity purification and microsequence analysis of the proteins in the doublet identified them as the two highly related nucleolar transcription factors, UBF1 and UBF2 (also known as the nucleolar autoantigen NOR-90). Immunoblot analysis confirmed that both proteins also interacted with the full-length CENP-C polypeptide with similar affinities. Double indirect immunofluorescence using monospecific antibodies demonstrated that a subset of CENP-C and UBF/NOR-90 is colocalized at nucleoli of interphase HeLa cells, suggesting that the in vitro interaction detected by affinity chromatography may reflect an interaction that occurs in vivo. We discuss the implications of these findings in terms of the properties of interphase centromeres and the role of the nucleolus in scleroderma autoimmunity.

Molecular cloning of an intronless gene for the hamster centromere antigen CENP-B

Centromere protein B (CENP-B) is a DNA-binding protein present at both active and inactive centromeres. It was first localized at the kinetochore region by human autoimmune sera from CREST patients. Using a previously identified human cDNA we have isolated a genomic clone containing the complete hamster CENP-b intronless coding sequence. At the nucleotide level it was found to possess a high degree of homology with the human and mouse CENP-B genes, being 75% and 90% respectively. This codes for 606 amino acid residues, which represent seven more than the human and mouse centromeric proteins. Hamster CENP-B protein analysis revealed at the N-terminal region a 133 amino acid fragment of 100% homology to the DNA binding motif identified previously for the human autoantigen. Expression of hamster CENP-B during the cell cycle was analyzed by using a specific antiCENP-B serum generated against the C-terminal conserved region. These data indicate that CENP-B is highly conserved and it represents a universal component of the centromere structure and function in mammals.

The centromere: hub of chromosomal activities

Centromeres are the structures that direct eukaryotic chromosome segregation in mitosis and meiosis. There are two major classes of centromeres. Point centromeres, found in the budding yeasts, are compact loci whose constituent proteins are now beginning to yield to biochemical analysis. Regional centromeres, best described in the fission yeast Schizosaccharomyces pombe, encompass many kilobases of DNA and are packaged into heterochromatin. Their associated proteins are as yet poorly understood. In addition to providing the site for microtubule attachment, centromeres also have an important role in checkpoint regulation during mitosis.

Theoretical analyses of chiasmata using a novel chiasma graph method applied to Chinese hamsters, mice, and dog

Some basic concepts of chiasma (including chiasma distribution, chiasma frequency, interstitial and terminal chiasmata, and chiasma interference) are reexamined theoretically in the light of gene shuffling, and a new method for chiasma analysis termed the chiasma graph is proposed. Chiasma graphs are developed for three mammals with greatly different chromosome numbers: Chinese hamster (with n = 11), mice (n = 20), and a dog (n = 39). The results demonstrate that interstitial chiasmata can contribute both to gene shuffling and to the binding of bivalents, but that so-called terminal chiasmata are in fact mostly achiasmatic terminal associations, the main function of which is to bind bivalents. For this reason, terminal chiasmata should be excluded when chiasma frequency is estimated. It is also demonstrated that interstitial chiasmata distribute on bivalents randomly and uniformly, except at the centromere and telomere. Interference distance fluctuates almost randomly above a minimum value equivalent to about 1.8% of total bivalent length at diakinesis. These results indicate that chiasma formation in mammals is principally a random event. The demonstrated minimum interference distance seems consistent with the polymerization model for chiasma formation. Some cytological aspects of crossing-over are discussed with reference to the minimum interaction theory for eukaryotic chromosome evolution.

Binding affinities of highly repetitive DNA components for a nuclear scaffold protein from rat-ascites hepatoma cells

Separate samples of a self-ligated tandem dimer of a highly repetitive DNA component (369-bp HindIII fragment) from rat-ascites hepatoma nuclei were digested with different restriction enzymes that cleave only once in the monomer. The resulting 369-bp sequence-permuted monomers showed anomalously slow gel electrophoretic mobility. Of them, the XmnI fragment had the slowest mobility. This suggests that bending of the helix axis is the strongest in this fragment. Our previous work has shown that such a repetitive bent DNA has selective affinities for two nuclear scaffold proteins from rat liver that have molecular weights of 123,000 and 130,000 Hibino et al. (1992) Biochem. Biophys. Res. Commun., 184, 853-858; Hibino et al. (1993) Biochim. Biophys. Acta, 1174, 162-170). In the present experiment, it has been found that the nuclear scaffold fraction from rat-ascites hepatoma cells does not contain these proteins, but does have a repetitive bent DNA-binding protein that has a molecular weight of about 230,000. These results imply that there is some difference in the structure of nuclear DNA attachment region between rat liver and the hepatoma.

Cosmid assembly and anchoring to human chromosome 21

A human chromosome 21-specific cosmid library from the Lawrence Livermore National Laboratory has been analyzed by two complementary methods, fingerprinting and hybridization; 40% coverage of the entire chromosome 21 has been achieved. To prepare a contig pool, approximately 9300 cosmid clones randomly selected from the library were fingerprinted and automatically assembled into 467 overlapping sets by the fluorescence-tagged restriction fragment method. The average size of the overlapping sets was 9.5 cosmids with minimal tiling paths consisting of 5.4 cosmids with a 10-kb extension each. However, as many as 10% of overlaps within members were estimated to be false. For regional localization, we hybridized gridded arrays of cosmids with inter-Alu-PCR probes obtained from YAC clones and somatic cell hybrids and assigned 592 cosmids to 26 subregions of 21q. Of these, 371 clones were incorporated into 139 contigs, anchoring the total 1864 cosmids to the subregion. The remaining 221 clones were mapped as orphans. To correlate the cytogenetic, YAC, and cosmid maps on 21q, the translocation breakpoints of the chromosomes contained in the somatic cell hybrids were mapped with respect to the STS content of the YACs. From the gene cluster regions, 176 ribosomal and 25 alphoid clones were isolated by hybridization. Together, these sets of anchored contigs and cosmids will provide a valuable resource for construction of a high-resolution map and for isolation of genes of interest from chromosome 21.

Isolation of chromosome-associated proteins from Drosophila melanogaster that bind a human centromeric DNA sequence

The molecular mechanism involved in packaging centromeric heterochromatin is still poorly understood. CENP-B, a centromeric protein present in human cells, is though to be involved in this process. This is a DNA-binding protein that localizes to the central domain of the centromere of human and mouse chromosomes due to its association with the 17-bp CENP-B box sequence. We have designed a biochemical approach to search for functional homologues of CENP-B in Drosophila melanogaster. This strategy relies upon the use of DNA fragments containing the CENP-B box to identify proteins that specifically bind this sequence. Three polypeptides were isolated by nuclear protein extraction, followed by sequential ion exchange columns and DNA affinity chromatography. All three proteins are present in the complex formed after gel retardation with the human alphoid satellite DNA that contains the CENP-B box. Footprinting analysis reveals that the complex occupies both strands of the CENP-B box, although it is still unclear which of the polypeptides actually makes contact with the DNA. Localization of fluorescein-labeled proteins after microinjection into early Drosophila embryos shows that they associate with condensed chromosomes. Immunostaining of embryos with a polyclonal serum made against all three polypeptides also shows chromosomal localization throughout mitosis. During metaphase and anaphase the antigens appear to localize preferentially to centromeric heterochromatin. Immunostaining of neuroblasts chromosome spreads confirmed these results, though some staining of chromosomal arms is also observed. The data strongly suggests that the polypeptides we have identified are chromosomal binding proteins that accumulate mainly at the centromeric heterochromatin. Furthermore, DNA binding assays clearly indicate that they have a high specific affinity for the human CENP-B box. This would suggest that at least one of the three proteins isolated might be a functional homologue of the human CENP-B.

Functional cloning of centromere protein B (CENP-B) box-enriched alphoid DNA repeats utilizing the sequence-specific DNA binding activity of human CENP-B in vitro

The centromere is a distinctive portion of the chromosome consisting of 'centromere DNA' and 'centromere proteins'. Recently, a direct molecular interaction was discovered between human centromere protein B (CENP-B) and human centromeric alphoid repeats. This enabled us to isolate the CENP-B-targeted centromeric DNA sequences by positively utilizing the biologic activity of CENP-B in vitro. In the previous model experiment, we found that oligonucleotides covering the CENP-B binding sequences were enriched by the DNA immunoprecipitation procedure. Here we apply the same technique to the direct isolation of a functional part of human centromeric DNA from a genomic DNA library. Restriction digestion of two isolated clones showed the typical repeating pattern of an alphoid family that is known to localize at the centromeric region of all human chromosomes. Sequence analysis showed that these two clones frequently contain the authentic CENP-B binding motif, CTTCGTTGGAAACGGGA, or a new one with one base replaced, CTTCGTTGGAAACGGGT. The frequent distribution of these motifs suggests that the isolated sequences are directly involved in the organization of centromeric heterochromatin at the primary constriction in conjunction with CENP-B.

Cloning and molecular characterization of a novel chromosome specific centromere sequence of Chinese hamster

We isolated and characterized the first chromosome-specific satellite DNA (HC2sat) of Chinese hamster. This novel satellite was localized to the pericentric region of hamster chromosome 2. The 2.8 kb long repeat unit of HC2sat was identified and two such units were sequenced. Extended short range periodicity could not be revealed in repeat units. These elements are amongst the largest satellite repeat units reported from mammals to date. HC2sat is a major constituent of the pericentric region of CHO chromosome 2 representing a 7-14 Mb long DNA segment. Studies of long range organization of HC2sat indicated that the formation of the satellite array might occur in different phases and involved different amplification mechanisms.

Enhanced repair endonuclease activities from radiation-arrested G2 phase mammalian cells

HeLa cells arrested in G2 phase 22 h after receiving 11.5 Gy gamma-radiation contained 3.6-fold more EDTA-resistant DNA repair endonuclease activity than unirradiated cells. Enzyme activity was determined by measuring the release of fragments from an irradiated repetitive alpha DNA substrate or from synthetic substrates containing a single modified base, 8-oxoguanine (8-oxo-G), a major radiation product. It appeared that the radiation-induced enhanced repair activity in some cells might be a feature of radiation-induced G2 arrest. Indeed, unirradiated G2 HeLa cells that had been synchronized by double thymidine block contained 3-7-fold more endonuclease activity than G1 or S-phase cells. Similarly, two of four other cell lines tested exhibited elevated repair endonuclease activity in G2. However, all six cell lines tested exhibited radiation-enhanced repair endonuclease activity. Therefore, the underlying mechanism for radiation enhancement of enzyme activity remains to be clarified and does not seem to be completely accounted for as a consequence of G2 arrest. The results showed different substrate specificities among cell lines as well as differences during the cell cycle of individual cell lines. Repair endonuclease activity from all cell lines which we have tested were associated with 60-70 kDa proteins from Superose 12 columns. Since reports from other laboratories have described several different DNA repair activities in 50-70 kDa Superose 12 fractions, it seems possible that the DNA repair enzymes may be associated in a repairosome structure.