Biomedical subjects

R Kellum

Publications and source records attributed to R Kellum.

11 recordsLinked to original sources

PubMed2003

HP1 complexes and heterochromatin assembly.

Since its discovery almost two decades ago, heterochromatin protein 1 (HP1) has emerged as a major player in the transcriptional regulation of both heterochromatic and euchromatic genes as well as the mechanics of chromosome segregation and the functional and structural organization of the interphase nucleus. Recent years have brought the identification of a myriad of HP1-interacting proteins. Each of these is discussed in relationship to its role in heterochromatin assembly and HP1 function. The breadth of functions represented by HP1-interacting proteins testifies to its pivotal role in the daily operations of the nucleus.

Animals↗

PubMed2001

Drosophila heterochromatin protein 1 (HP1)/origin recognition complex (ORC) protein is associated with HP1 and ORC and functions in heterochromatin-induced silencing.

Heterochromatin protein 1 (HP1) is a conserved component of the highly compact chromatin of higher eukaryotic centromeres and telomeres. Cytogenetic experiments in Drosophila have shown that HP1 localization into this chromatin is perturbed in mutants for the origin recognition complex (ORC) 2 subunit. ORC has a multisubunit DNA-binding activity that binds origins of DNA replication where it is required for origin firing. The DNA-binding activity of ORC is also used in the recruitment of the Sir1 protein to silence nucleation sites flanking silent copies of the mating-type genes in Saccharomyces cerevisiae. A fraction of HP1 in the maternally loaded cytoplasm of the early Drosophila embryo is associated with a multiprotein complex containing Drosophila melanogaster ORC subunits. This complex appears to be poised to function in heterochromatin assembly later in embryonic development. Here we report the identification of a novel component of this complex, the HP1/ORC-associated protein. This protein contains similarity to DNA sequence-specific HMG proteins and is shown to bind specific satellite sequences and the telomere-associated sequence in vitro. The protein is shown to have heterochromatic localization in both diploid interphase and mitotic chromosomes and polytene chromosomes. Moreover, the gene encoding HP1/ORC-associated protein was found to display reciprocal dose-dependent variegation modifier phenotypes, similar to those for mutants in HP1 and the ORC 2 subunit.

Amino Acid Sequence↗

PubMed1998

Distinct cytoplasmic and nuclear fractions of Drosophila heterochromatin protein 1: their phosphorylation levels and associations with origin recognition complex proteins.

The distinct structural properties of heterochromatin accommodate a diverse group of vital chromosome functions, yet we have only rudimentary molecular details of its structure. A powerful tool in the analyses of its structure in Drosophila has been a group of mutations that reverse the repressive effect of heterochromatin on the expression of a gene placed next to it ectopically. Several genes from this group are known to encode proteins enriched in heterochromatin. The best characterized of these is the heterochromatin-associated protein, HP1. HP1 has no known DNA-binding activity, hence its incorporation into heterochromatin is likely to be dependent upon other proteins. To examine HP1 interacting proteins, we isolated three distinct oligomeric species of HP1 from the cytoplasm of early Drosophila embryos and analyzed their compositions. The two larger oligomers share two properties with the fraction of HP1 that is most tightly associated with the chromatin of interphase nuclei: an underphosphorylated HP1 isoform profile and an association with subunits of the origin recognition complex (ORC). We also found that HP1 localization into heterochromatin is disrupted in mutants for the ORC2 subunit. These findings support a role for the ORC-containing oligomers in localizing HP1 into Drosophila heterochromatin that is strikingly similar to the role of ORC in recruiting the Sir1 protein to silencing nucleation sites in Saccharomyces cerevisiae.

Amino Acid Sequence↗

PubMed1998

Chromatin boundaries: punctuating the genome.

Transcriptional enhancers are constrained to act within domains defined by boundary elements. How these elements work is a mystery. A recent study emphasizes their autonomous activity; another emphasizes their dependence on nuclear organization. Both effects need to be accounted for by any successful model.

Animals↗

PubMed1997

Association of the origin recognition complex with heterochromatin and HP1 in higher eukaryotes.

The origin recognition complex (ORC) is required to initiate eukaryotic DNA replication and also engages in transcriptional silencing in S. cerevisiae. We observed a striking preferential but not exclusive association of Drosophila ORC2 with heterochromatin on interphase and mitotic chromosomes. HP1, a heterochromatin-localized protein required for position effect variegation (PEV), colocalized with DmORC2 at these sites. Consistent with this localization, intact DmORC and HP1 were found in physical complex. The association was shown biochemically to require the chromodomain and shadow domains of HP1. The amino terminus of DmORC1 contained a strong HP1-binding site, mirroring an interaction found independently in Xenopus by a yeast two-hybrid screen. Finally, heterozygous DmORC2 recessive lethal mutations resulted in a suppression of PEV. These results indicate that ORC may play a widespread role in packaging chromosomal domains through interactions with heterochromatin-organizing factors.

Amino Acid Sequence↗

PubMed1995

Heterochromatin protein 1 distribution during development and during the cell cycle in Drosophila embryos.

Heterochromatin protein 1 (HP1) was initially discovered as a protein that is associated with the heterochromatin at the chromocenter of polytene chromosomes in Drosophila larval salivary glands. In this paper we investigate the localization of heterochromatin protein 1 in the diploid nuclei of Drosophila embryos. We focus on its association with the interphase heterochromatin in fixed embryos before and during cycle 14, the developmental time at which heterochromatin becomes most conspicuous, and also follow its localization during mitosis. The GAGA transcription factor was recently shown to be localized at sequences within alpha-heterochromatin in pre-cycle 14 embryos, and an antibody against this protein serves as a convenient marker for these sequences. We find an enrichment of heterochromatin protein 1 in the intensely DAPI-staining regions near the apical surface of nuclear cycle 10 embryos. At this stage GAGA factor is localized into punctate structures in this same region. This enrichment for HP1 is markedly increased during nuclear cycle 14. Surprisingly, whereas GAGA factor retains its association with the heterochromatin throughout the cell cycle, a significant fraction of HP1 is dispersed throughout the spindle around the segregating chromosomes during mitosis. This dispersed pool of heterochromatin protein 1 was observed during mitosis in both early and late Drosophila embryos and in an analysis of a bacterially produced 6x histidine-heterochromatin protein 1 fusion protein injected into living Drosophila embryos. When Drosophila tissue culture cells were prepared by a method which removes soluble protein and avoids fixation of the mitotic chromosomes, an enrichment for heterochromatin protein 1 in the heterochromatin of the chromosomes was discovered also.

Animals↗

PubMed1995

Heterochromatin protein 1 is required for correct chromosome segregation in Drosophila embryos.

Heterochromatin protein 1 is associated with centromeric heterochromatin in Drosophila, mice, and humans. Loss of function mutations in the gene encoding heterochromatin protein 1 in Drosophila, Suppressor of variegation2-5, decrease the mosaic repression observed for euchromatic genes that have been juxtaposed to centromeric heterochromatin. These heterochromatin protein 1 mutations not only suppress this position-effect variegation, but also cause recessive embryonic lethality. In this study, we analyze the latter phenotype in the hope of gaining insight into heterochromatin function. In our analyses of four alleles of Suppressor of variegation2-5, the lethality was found to be associated with defects in chromosome morphology and segregation. While some of these defects are seen throughout embryonic development, both the frequency and severity of the defects are greatest between cycles 10 and 14 when zygotic transcription of the Suppressor of variegation2-5 gene apparently begins. By this time in development, heterochromatin protein 1 levels are diminished by four-fold in a quarter of the embryos produced by parents that are both heterozygous for a null allele (Suppressor of variegation2-5(05)). In a live analysis of the phenotype, we find prophase to be lengthened by more than two-fold in Suppressor of variegation2-5(05) mutant embryos with subsequent defects in chromosome segregation. The elongated prophase suggests that the segregation phenotype is a consequence of defects in events that occur during prophase, either in chromosome condensation or kinetochore assembly or function. Immunostaining with an antibody against a centromerespecific antigen indicates that the kinetochores of most chromosomes are functional. The immunostaining results are more consistent with defects in chromosome condensation being responsible for the segregation phenotype.

Alleles↗

PubMed1994

The Drosophila GAGA transcription factor is associated with specific regions of heterochromatin throughout the cell cycle.

In virtually all eukaryotes the centromeric regions of chromosomes are composed of heterochromatin, a specialized form of chromatin that is rich in repetitive DNA sequences and is transcriptionally relatively silent. The Drosophila GAGA transcription factor binds to GA/CT-rich sequences in many Drosophila promoters, where it activates transcription, apparently by locally altering chromatin structure and allowing other transcription factors access to the DNA. Here we report the paradoxical finding that GAGA factor is associated with specific regions of heterochromatin at all stages of the cell cycle. A subset of the highly repetitive DNA sequences that make up the bulk of heterochromatin in D. melanogaster are GA/CT-rich and we find a striking correlation between the distribution of GAGA factor and this class of repeat. We propose that GAGA factor binds directly to these repeats and may thereby play a role in modifying heterochromatin structure in these regions. Our observations demonstrate for the first time that a transcriptional regulator can associate with specific DNA sequences in a fully condensed mitotic chromosome. This may help explain how the distinctive character of a committed or differentiated cell can be maintained during cell proliferation.

Animals↗

PubMed1992

A group of scs elements function as domain boundaries in an enhancer-blocking assay.

Chromosomes of higher eukaryotes are thought to be organized into a series of discrete and topologically independent higher-order domains. In addition to providing a mechanism for chromatin compaction, these higher-order domains are thought to define independent units of gene activity. Implicit in most models for the folding of the chromatin fiber are special nucleoprotein structures, the domain boundaries, which serve to delimit each higher-order chromosomal domain. We have used an "enhancer-blocking assay" to test putative domain boundaries for boundary function in vivo. This assay is based on the notion that in delimiting independent units of gene activity, domain boundaries should be able to restrict the scope of activity of enhancer elements to genes which reside within the same domain. In this case, interposing a boundary between an enhancer and a promoter should block the action of the enhancer. In the experiments reported here, we have used the yolk protein-1 enhancer element and an hsp70 promoter:lacZ fusion gene to test putative boundary DNA segments for enhancer-blocking activity. We have found that several scs-like elements are capable of blocking the action of the yp-1 enhancer when placed between it and the hsp70 promoter. In contrast, a MAR/SAR DNA segment and another spacer DNA segment had no apparent effect on enhancer activity.

Animals↗

PubMed1991

A position-effect assay for boundaries of higher order chromosomal domains.

Eukaryotic chromosomes are thought to be organized into a series of discrete higher order chromatin domains. This organization is believed to be important not only in the compaction of the chromatin fiber, but also in the utilization of genetic information. Each domain would define an independent unit of gene activity, insulated from the regulatory influences of adjacent domains. Critical to this model of chromosome organization and function are the domain boundaries: the special nucleoprotein structures that delimit each higher order domain and segregate the chromosome into units of independent gene activity. In the work reported here we have tested whether two putative domain boundaries, scs and scs', from the Drosophila 87A7 heat shock locus can establish a domain of independent gene activity in vivo and insulate against chromosomal position effects.

Animals↗

PubMed1984

Effects of cultivation gas phase on hydrogenase of the acetogen Clostridium thermoaceticum.

The effect of cultivation gas phase on the expression and activity of hydrogenase in heterotrophic cultures of Clostridium thermoaceticum was examined. Of the five gas phases tested, hydrogenase was maximal from cells cultivated under CO. Correlations were observed between the level of hydrogenase and the evolution of H2 by growing cultures. Activity stains of polyacrylamide gels revealed a single hydrogenase band in CO2 cells and multiple hydrogenase bands in CO cells.

Carbon Dioxide↗