A histone H3 methyltransferase controls DNA methylation in Neurospora crassa.

DNA methylation is involved in epigenetic processes such as X-chromosome inactivation, imprinting and silencing of transposons. We have demonstrated previously that dim-2 encodes a DNA methyltransferase that is responsible for all known cytosine methylation in Neurospora crassa. Here we report that another Neurospora gene, dim-5, is required for DNA methylation, as well as for normal growth and full fertility. We mapped dim-5 and identified it by transformation with a candidate gene. The mutant has a nonsense mutation in a SET domain of a gene related to histone methyltransferases that are involved in heterochromatin formation in other organisms. Transformation of a wild-type strain with a segment of dim-5 reactivated a silenced hph gene, apparently by 'quelling' of dim-5. We demonstrate that recombinant DIM-5 protein specifically methylates histone H3 and that replacement of lysine 9 in histone H3 with either a leucine or an arginine phenocopies the dim-5 mutation. We conclude that DNA methylation depends on histone methylation.     

Regulation of HP1-chromatin binding by histone H3 methylation and phosphorylation

Tri-methylation of histone H3 lysine 9 is important for recruiting heterochromatin protein 1 (HP1) to discrete regions of the genome, thereby regulating gene expression, chromatin packaging and heterochromatin formation. Here we show that HP1alpha, -beta, and -gamma are released from chromatin during the M phase of the cell cycle, even though tri-methylation levels of histone H3 lysine 9 remain unchanged. However, the additional, transient modification of histone H3 by phosphorylation of serine 10 next to the more stable methyl-lysine 9 mark is sufficient to eject HP1 proteins from their binding sites. Inhibition or depletion of the mitotic kinase Aurora B, which phosphorylates serine 10 on histone H3, causes retention of HP1 proteins on mitotic chromosomes, suggesting that H3 serine 10 phosphorylation is necessary for the dissociation of HP1 from chromatin in M phase. These findings establish a regulatory mechanism of protein-protein interactions, through a combinatorial readout of two adjacent post-translational modifications: a stable methylation and a dynamic phosphorylation mark.     

CpG methylation is maintained in human cancer cells lacking DNMT1

Hypermethylation is associated with the silencing of tumour susceptibility genes in several forms of cancer; however, the mechanisms responsible for this aberrant methylation are poorly understood. The prototypic DNA methyltransferase, DNMT1, has been widely assumed to be responsible for most of the methylation of the human genome, including the abnormal methylation found in cancers. To test this hypothesis, we disrupted the DNMT1 gene through homologous recombination in human colorectal carcinoma cells. Here we show that cells lacking DNMT1 exhibited markedly decreased cellular DNA methyltransferase activity, but there was only a 20% decrease in overall genomic methylation. Although juxtacentromeric satellites became significantly demethylated, most of the loci that we analysed, including the tumour suppressor gene p16INK4a, remained fully methylated and silenced. These results indicate that DNMT1 has an unsuspected degree of regional specificity in human cells and that methylating activities other than DNMT1 can maintain the methylation of most of the genome.     

C-banding pattern and nucleolar organizer regions of Cynoglossus semilaevis Günther, 1873

The C-banding pattern and nucleolar organizer regions of the metaphase chromosomes of Cynoglossus semilaevis are reported. The interstitial regions in all chromosomes including the pair of sex chromosomes had positive C-bands, and the 6th and 12th pairs of chromosomes were entirely stained in most cases. Some chromosomes such as the 1st, 2nd, 7th, 8th, 9th and 10th pairs showed C-bands at centromeric or distal ends. The C-banding heretochromatin occupies 30.03% of the total chromosome surface in C. semilaevis, which is similar to that of amphibians such as Mixophyes fasciolatus (30. 2% ) and M. schevilli (20. 7% ), but is rather lower than that of cephalochordate Branchiostoma belcheri (54. 3 % ). Silver staining revealed a single pair of nucleolar organizer regions ( NORs) located in the telomeric regions of Chromosome 2. The association of NORs with heterochromatin observed in vertebrates also occurs in C. semilaevis as the telomeric regions of Chromosome 2 are always stained positively with C-banding.     

Cloning defined regions of the human genome by microdissection of banded chromosomes and enzymatic amplification

The molecular analysis of many genetic diseases requires the isolation of probes for defined human chromosome regions. Existing techniques such as the screening of chromosome-specific libraries, subtractive DNA cloning and chromosome jumping are either tedious or not generally applicable. Microdissection and microcloning has successfully been applied to various chromosome regions in Drosophila and mouse, but conventional microtechniques are too coarse and inefficient for analysis of the human genome. Because microdissection has previously been used on unbanded chromosomes only, cell lines in which the chromosome of interest could be identified without banding had to be used. At least one hundred chromosomes were needed for dissection and lambda vectors used to achieve maximum cloning efficiency. Recombinant phage clones are, however, more difficult to characterize than plasmid clones. Here we describe the dissection of the Langer-Giedion syndrome region on chromosome 8 from GTG-banded metaphase chromosomes (G-banding with trypsin-Giemsa) and the universal enzymatic amplification of the dissected DNA. Eighty per cent of clones from this library (total yield 20,000) identify single-copy DNA sequences. Fifty per cent of clones detect deletions in two patients with Langer-Giedion syndrome. Although the other clones have not yet been mapped, this result demonstrates that thousands of region-specific probes can be isolated within ten days.     

Genomic imprinting determines methylation of parental alleles in transgenic mice

Mouse embryogenesis relies on the presence of both the maternal and the paternal genome for development to term. It has been proposed that specific modifications are imprinted onto the chromosomes during gametogenesis; these modifications are stably propagated, and their expression results in distinct and complementary contributions of the two parental genomes to the development of the embryo and the extraembryonic membranes. Genetic data further suggest that a substantial proportion of the genome could be subject to chromosomal imprinting, the molecular nature of which is unknown. We used random DNA insertions in transgenic mice to probe the genome for modified regions. The DNA methylation patterns of transgenic alleles were compared after transmission from mother or father in seven mouse strains carrying autosomal insertions of the same transgenic marker. One of these loci showed a clear difference in DNA methylation specific for its parental origin, with the paternally inherited copy being relatively undermethylated. This difference was observed in embryos on day 10 of gestation, but not in their extraembryonic membranes. Moreover, the methylation pattern was faithfully reversed upon each germline transmission to the opposite sex. Our findings provide evidence for heritable molecular differences between maternally and paternally derived alleles on mouse chromosomes.     

C-banding pattern and nucleolar organizer regions of Cynoglossus semilaevis Günther, 1873

The C-banding pattern and nucleolar organizer regions of the metaphase chromosomes of Cynoglossus semilaevis are reported. The interstitial regions in all chromosomes including the pair of sex chromosomes had positive C-bands, and the 6th and 12th pairs of chromosomes were entirely stained in most cases. Some chromosomes such as the 1st, 2nd, 7th, 8th, 9th and 10th pairs showed C-bands at centromeric or distal ends. The C-banding heretochromatin occupies 30.03% of the total chromosome surface in C. semilaevis,which is similar to that of amphibians such as Mixophyes fasciolatus (30.2%) and M. schevilli (20.7%), but is rather lower than that of cephalochordate Branchiostoma belcheri (54.3%). Silver staining revealed a single pair of nucleolar organizer regions (NORs) located in the telomeric regions of Chromosome 2. The association of NORs with heterochromatin observed in vertebrates also occurs in C. semilaevis as the telomeric regions of Chromosome 2 are always stained positively with C-banding.     

DNA triplet repeats mediate heterochromatin-protein-1-sensitive variegated gene silencing

Gene repression is crucial to the maintenance of differentiated cell types in multicellular organisms, whereas aberrant silencing can lead to disease. The organization of DNA into chromatin and heterochromatin is implicated in gene silencing. In chromatin, DNA wraps around histones, creating nucleosomes. Further condensation of chromatin, associated with large blocks of repetitive DNA sequences, is known as heterochromatin. Position effect variegation (PEV) occurs when a gene is located abnormally close to heterochromatin, silencing the affected gene in a proportion of cells. Here we show that the relatively short triplet-repeat expansions found in myotonic dystrophy and Friedreich's ataxia confer variegation of expression on a linked transgene in mice. Silencing was correlated with a decrease in promoter accessibility and was enhanced by the classical PEV modifier heterochromatin protein 1 (HP1). Notably, triplet-repeat-associated variegation was not restricted to classical heterochromatic regions but occurred irrespective of chromosomal location. Because the phenomenon described here shares important features with PEV, the mechanisms underlying heterochromatin-mediated silencing might have a role in gene regulation at many sites throughout the mammalian genome and modulate the extent of gene silencing and hence severity in several triplet-repeat diseases.     

C-banding pattern and nucleolar organizer regions of Cynoglossus semilaevis Günther, 1873

The C-banding pattern and nucleolar organizer regions of the metaphase chromosomes of Cynoglossus semilaevis are reported. The interstitial regions in all chromosomes including the pair of sex chromosomes had positive C-bands, and the 6th and 12th pairs of chromosomes were entirely stained in most cases. Some chromosomes such as the 1st, 2nd, 7th, 8th, 9th and 10th pairs showed C-bands at centromeric or distal ends. The C-banding heretochromatin occupies 30.03% of the total chromosome surface in C. semilaevis, which is similar to that of amphibians such as Mixophyes fasciolatus (30.2%) and M. schevilli (20.7%), but is rather lower than that of cephalochordate Branchiostoma belcheri (54.3%). Silver staining revealed a single pair of nucleolar organizer regions (NORs) located in the telomeric regions of Chromosome 2. The association of NORs with heterochromatin observed in vertebrates also occurs in C. semilaevis as the telomeric regions of Chromosome 2 are always stained positively with C-banding.     

C-banding pattern and nucleolar organizer regions of Cynoglossus semilaevis Günther, 1873

The C-banding pattern and nucleolar organizer regions of the metaphase chromosomes of Cynoglossus semilaevis are reported. The interstitial regions in all chromosomes including the pair of sex chromosomes had positive C-bands, and the 6th and 12th pairs of chromosomes were entirely stained in most cases. Some chromosomes such as the 1st, 2nd, 7th, 8th, 9th and 10th pairs showed C-bands at centromeric or distal ends. The C-banding heretochromatin occupies 30.03% of the total chromosome surface in C. semilaevis, which is similar to that of amphibians such as Mixophyes fasciolatus (30.2%) and M. schevilli (20.7%), but is rather lower than that of cephalochordate Branchiostoma belcheri (54.3%). Silver staining revealed a single pair of nucleolar organizer regions (NORs) located in the telomeric regions of Chromosome 2. The association of NORs with heterochromatin observed in vertebrates also occurs in C. semilaevis as the telomeric regions of Chromosome 2 are always stained positively with C-banding.     

济南花鸡染色体研究

对山东省培育的家鸡优良品种济南花鸡的三个品系：草花鸡、麻花鸡和无翼鸡作了染色体核型分析，进行了染色体的显带处理，探讨了Ｇ－、Ｃ－带型和核仁组织者的分布，并进行了比较研究．     

两种鱼的复制带显带研究

采用活体直接注射ＢｒｄＵ的方法,对黄颡鱼和鲶鱼的染色体复制带进行了研究．结果发现,采用ＢｒｄＵ－Ｈｏｅｃｈｓｔ－Ｇｉｅｍｓａ方法,获得了这两种鱼的复制带图像,带纹清晰稳定．复制带可分早、中、晚三个时期,从早期到晚期,带纹逐渐减少;常染色质早复制,异染色质晚复制,复制中期可看到着丝粒区、端粒区和居间区,其中特别是ＮＯＲｓ处浅染．根据试验结果,对鱼类染色体的复制带特征进行了讨论     

Sequence and analysis of chromosome 5 of the plant Arabidopsis thaliana

The genome of the model plant Arabidopsis thaliana has been sequenced by an international collaboration, The Arabidopsis Genome Initiative. Here we report the complete sequence of chromosome 5. This chromosome is 26 megabases long; it is the second largest Arabidopsis chromosome and represents 21% of the sequenced regions of the genome. The sequence of chromosomes 2 and 4 have been reported previously and that of chromosomes 1 and 3, together with an analysis of the complete genome sequence, are reported in this issue. Analysis of the sequence of chromosome 5 yields further insights into centromere structure and the sequence determinants of heterochromatin condensation. The 5,874 genes encoded on chromosome 5 reveal several new functions in plants, and the patterns of gene organization provide insights into the mechanisms and extent of genome evolution in plants.