Active genes are tri-methylated at K4 of histone H3

Lysine methylation of histones in vivo occurs in three states: mono-, di- and tri-methyl. Histone H3 has been found to be di-methylated at lysine 4 (K4) in active euchromatic regions but not in silent heterochromatic sites. Here we show that the Saccharomyces cerevisiae Set1 protein can catalyse di- and tri-methylation of K4 and stimulate the activity of many genes. Using antibodies that discriminate between the di- and tri-methylated state of K4 we show that di-methylation occurs at both inactive and active euchromatic genes, whereas tri-methylation is present exclusively at active genes. It is therefore the presence of a tri-methylated K4 that defines an active state of gene expression. These findings establish the concept of methyl status as a determinant for gene activity and thus extend considerably the complexity of histone modifications.     

Rearrangements of the cellular p53 gene in erythroleukaemic cells transformed by Friend virus

There is now good evidence that the cellular protein, p53, is involved in the transformation process, although its precise role is unknown. It was reported recently that expression of the p53 gene can immortalize cells and that the p53 gene can replace the myc oncogene in a myc-ras immortalization/transformation assay. We have investigated whether p53 is involved in the progression towards the neoplastic state in vivo and report here that erythroleukaemic cell lines transformed by different isolates of Friend leukaemia virus show altered expression of the cellular p53 gene. High levels of p53 protein are found in certain lines, but the protein is undetectable in others. This heterogeneity in p53 gene expression is associated with heterogeneity in tumorigenicity. We demonstrate that genomic rearrangements are responsible for p53 gene inactivation in these cell lines and that they occur in vivo during the natural progression of Friend virus-induced erythroleukaemia.     

Assortative mating in insects: sexual selection vs. sampling artifact

In this report we analyze patterns of size-assortative mating in an insect genus by (1) describing the consistency and strength of assortative mating among populations of a species and among species of a genus, and (2) determining the role of sampling artifacts in generating this nonrandom pattern of mating. Our data consist of 18 samples of soldier beetles (8 species of the genus Chauliognathus ) from 17 sites within the southwestern United States. Of the 18 samples, just 6 exhibited significant correlations between elytron lengths of mating pairs. We show how these and other significant correlations are generated by 3 kinds of sampling artifacts: mixed-species samples, mixed-population samples, and mixed-microsite samples.     

Retrovirus transfer of a bacterial gene into mouse haematopoietic progenitor cells

The haematopoietic system is made up of a hierarchy of cells with different developmental, functional and proliferative capacities. Although cellular diversity appears to arise from the commitment and maturation of stem cells, the molecular basis for this differentiation process is unknown. The introduction of cloned DNA sequences into haematopoietic progenitor cells would provide a novel approach for studying this differentiating in vivo system. One laboratory has reported DNA-mediated transfer of genes into mouse bone marrow cells. However, retroviruses offer a number of advantages over DNA-mediated gene transfer procedures, including high efficiency infection of a wide range of cell types in vitro and in vivo, stable and low copy integration into the host chromosome, and a defined integrated provirus structure. For these reasons recombinant DNA techniques have been utilized to construct high efficiency retrovirus vectors expressing foreign genes. We demonstrate here, using such a retrovirus vector, the transfer of a dominant selectable drug-resistance gene into defined classes of mouse haematopoietic progenitor cells. These observations should facilitate the development of molecular genetic approaches to fundamental and clinical problems in haematopoiesis.     

Molecular identification of a human DNA repair gene following DNA-mediated gene transfer

Although it has long been evident that the response of eukaryotes to DNA damaging agents is determined by the effectiveness of a variety of DNA repair systems, there is little detailed knowledge of the nature of these systems or the genes which control them. In humans, a number of hereditary conditions, including xeroderma pigmentosum, ataxia telangiectasia and Fanconi's anaemia, exhibit increased sensitivity to a variety of DNA damaging agents and a predisposition to cancer, suggesting a defect in some aspect of DNA repair. This report describes the identification of a human DNA repair gene following DNA-mediated gene transfer into Chinese hamster ovary (CHO) mutant cells, that like xeroderma pigmentosum cells, are sensitive to a variety of DNA damaging agents and are defective in the initial incision step of DNA repair. The resulting transformants exhibit normal resistance to DNA damaging agents and independent transformants demonstrate a common set of human DNA sequences associated with a human DNA repair gene. These observations provide the basis for the isolation and characterization of the human genes responsible for DNA repair.     

The proto-oncogene c-kit encoding a transmembrane tyrosine kinase receptor maps to the mouse W locus

Mice carrying mutations at the W locus located on chromosome 5 are characterized by severe macrocytic anaemia, lack of hair pigmentation and sterility. Mutations at this locus appear to affect the proliferation and/or migration of cells during early embryogenesis and result in an intrinsic defect in the haematopoietic stem cell hierarchy. An understanding of the molecular basis of the complex and pleiotropic phenotype in W mutant mice would thus provide insights into the important developmental processes of gametogenesis, melanogenesis and haematopoiesis. Here we show that the mouse mutant W has a deletion of the c-kit proto-oncogene. Interspecific backcross analysis demonstrates that the W locus is very tightly linked to c-kit and that the two loci cannot be segregated at this level of analysis. c-kit is the cellular homologue of the oncogene v-kit of the HZ4 feline sarcoma virus and encodes a transmembrane protein tyrosine kinase receptor that is structurally similar to the receptors for colony-stimulating factor-1 (CSF-1) and platelet derived growth factor. The co-localization of c-kit with W provides a molecular entry into this important region of the mouse genome. In addition, these observations provide the first example of a germ-line mutation in a mammalian proto-oncogene and implicate the c-kit gene as a candidate for the W locus.     

Assortative mating in soldier beetles (Cantharidae: Chaulignathus ): test of the mate-choice hypothesis

Soldier beetles of 2 species, Chauliognathus basalis and C. deceptus , were examined to test the Crespi hypothesis that positive assortative mating by size is caused by mate choices. Specifically, we tested the prediction that if mate choice involves choosing the largest mate available, then mating individuals will be larger than nonmating individuals. Four samples were taken, at different times during the mating season, from each of 2 sites. Each sample consisted of mating pairs, nonmanting males, and nonmating females. Some of the samples contained beetles of both species; others contained beetles of a single species. For each gender elytron lengths of mating individuals were compared with elytron lengths of nonmating individuals. We found no effect of mating status (mating vs. nonmating) on elytron lengths in samples that exhibited assertive mating (which occurs where 2 species coexist). Surprisingly, we found a consistent effect of mating status on elytron lengths in samples that did not exhibit assortative mating (which occurs where only 1 species exists). Our results do not support the mate-choice hypothesis. Instead, mate choice and assortative mating appear to be alternative mating patterns in which mate choice occurs where a single species exists and assortative mating occurs where 2 species coexist.     

Dicer is essential for mouse development

To address the biological function of RNA interference (RNAi)-related pathways in mammals, we disrupted the gene Dicer1 in mice. Loss of Dicer1 lead to lethality early in development, with Dicer1-null embryos depleted of stem cells. Coupled with our inability to generate viable Dicer1-null embryonic stem (ES) cells, this suggests a role for Dicer, and, by implication, the RNAi machinery, in maintaining the stem cell population during early mouse development.