Evidence for an Epigenetic Mechanism by which Hsp90 Acts as a Capacitor for Morphological Evolution

Morphological alterations have been shown to occur in Drosophila melanogaster when function of Hsp90 (heat shock 0-kDa protein 1α, encoded by Hsp83) is compromised during development1. Genetic selection maintains the altered phenotypes in subsequent generations1. Recent experiments have shown, however, that phenotypic variation still occurs in nearly isogenic recombinant inbred strains of Arabidopsis thaliana2. Using a sensitized isogenic D. melanogaster strain, iso-KrIf-1, we confirm this finding and present evidence supporting an epigenetic mechanism for Hsp90’s capacitor function, whereby reduced activity of Hsp90 induces a heritably altered chromatin state. The altered chromatin state is evidenced by ectopic expression of the morphogen wingless in eye imaginal discs and a corresponding abnormal eye phenotype, both of which are epigenetically heritable in subsequent generations, even when function of Hsp90 is restored. Mutations in nine different genes of the trithorax group that encode chromatin-remodeling proteins also induce the abnormal phenotype. These findings suggest that Hsp90 acts as a capacitor for morphological evolution through epigenetic and genetic mechanisms

Lack of increases in methylation at three CpG-rich genomic loci in non-mitotic adult tissues during aging

<p>Abstract</p> <p>Background</p> <p>Cell division occurs during normal human development and aging. Despite the likely importance of cell division to human pathology, it has been difficult to infer somatic cell mitotic ages (total numbers of divisions since the zygote) because direct counting of lifetime numbers of divisions is currently impractical. Here we attempt to infer relative mitotic ages with a molecular clock hypothesis. Somatic genomes may record their mitotic ages because greater numbers of replication errors should accumulate after greater numbers of divisions. Mitotic ages will vary between cell types if they divide at different times and rates.</p> <p>Methods</p> <p>Age-related increases in DNA methylation at specific CpG sites (termed "epigenetic molecular clocks") have been previously observed in mitotic human epithelium like the intestines and endometrium. These CpG rich sequences or "tags" start unmethylated and potentially changes in methylation during development and aging represent replication errors. To help distinguish between mitotic versus time-associated changes, DNA methylation tag patterns at 8–20 CpGs within three different genes, two on autosomes and one on the X-chromosome were measured by bisulfite sequencing from heart, brain, kidney and liver of autopsies from 21 individuals of different ages.</p> <p>Results</p> <p>Levels of DNA methylation were significantly greater in adult compared to fetal or newborn tissues for two of the three examined tags. Consistent with the relative absence of cell division in these adult tissues, there were no significant increases in tag methylation after infancy.</p> <p>Conclusion</p> <p>Many somatic methylation changes at certain CpG rich regions or tags appear to represent replication errors because this methylation increases with chronological age in mitotic epithelium but not in non-mitotic organs. Tag methylation accumulates differently in different tissues, consistent with their expected genealogies and mitotic ages. Although further studies are necessary, these results suggest numbers of divisions and ancestry are at least partially recorded by epigenetic replication errors within somatic cell genomes.</p

Pcl-PRC2 is needed to generate high levels of H3-K27 trimethylation at Polycomb target genes

PRC2 is thought to be the histone methyltransferase (HMTase) responsible for H3-K27 trimethylation at Polycomb target genes. Here we report the biochemical purification and characterization of a distinct form of Drosophila PRC2 that contains the Polycomb group protein polycomblike (Pcl). Like PRC2, Pcl-PRC2 is an H3-K27-specific HMTase that mono-, di- and trimethylates H3-K27 in nucleosomes in vitro. Analysis of Drosophila mutants that lack Pcl unexpectedly reveals that Pcl-PRC2 is required to generate high levels of H3-K27 trimethylation at Polycomb target genes but is dispensable for the genome-wide H3-K27 mono- and dimethylation that is generated by PRC2. In Pcl mutants, Polycomb target genes become derepressed even though H3-K27 trimethylation at these genes is only reduced and not abolished, and even though targeting of the Polycomb protein complexes PhoRC and PRC1 to Polycomb response elements is not affected. Pcl-PRC2 is thus the HMTase that generates the high levels of H3-K27 trimethylation in Polycomb target genes that are needed to maintain a Polycomb-repressed chromatin state

MILLIMETER-WAVE ROTATIONAL SPECTRUM OF VIBRATIONALLY EXCITED C3C_{3}H2H_{2}

1. P. Thaddeus, J.M. Vrtilek, and C.A. Gottlieb, Astrophys. J. 299, L63 (1985). 2. Y. Hirahara, A. Masuda, and K. Kawaguchi, J. Chem, Phys. 95, 3975 (1991).Author Institution: Division of Applied SciencesPure rotational transitions of $C_{2}H_{2}$, a three-membered carbene ring that is a ubiquitous interstellar molecule, were detected in the laboratory and in space in $1985.^{1}$ Following the recent observation of the $\nu_{2}$ mode at. $1277 cm^{-1}$ by infrared $spectroscopy,^{2}$ we observed millimeter-wave rotational transitions of the 1/3 mode and three other vibrational modes (one with B symmetry and two with A symmetry) in a dc glow discharge through allene and helium. The key to the identification of the vibrationally excited states was detection of closely spaced ortho-para doublets near 184 GHz within 3 GHz of the ground state transitions. Subsequently, further rotational lines in the range 150 to 385 GHz were identified, and accurate rotational and centrifugal constants for all four modes were determined. Although we are presently engaged in assigning these modes from comparison of inertial defects from experimental and ab initio force constants, conclusive assignment will require infrared spectroscopy

LASER-INDUCED-FLUORESCENCE DETECTION OF OVERTONE TRANSITIONS OF TWO HF CONTAINING COMPLEXES

Author Institution: Department of Chemistry, Harvard University; Department of Chemistry, Institute of Atomic and Molecular ScienceThe high-resolution laser-induced fluorescence spectra of the v = 3 excitation of the HF stretch in HFHC1 and $N_{2}HF$ molecules are measured. The complexes are formed in a super-sonic jet with HFHC1 in an argon expansion and $N_{2}HF$ in a nitrogen expansion. For HFHCl, a single band with $K = 0,\Delta K=0$ of both isotopic species, was observed; and for $N_{2}HF$, both the $\Sigma - \Sigma$ second overtone of pure HF stretch and the $\Pi - \Pi$ hot band of the $N_{2}$ bend were recorded. The HF submolecule was pumped from v = 0 to v = 3 by radiation from the TiSapphire ring laser operating in the 800 - 900 nm range. Both $\Delta v = 1$ and $\Delta v = 2$ emission of the HF fragment were observed by large area PbS and Ge detectors, respectively. The average linewidth of 300 MHz in this overtone region is approximately 30 times greater than at the fundamental. The linewidths of the vibration-rotation transitions of HFHCl show considerable variations with both Cl isotope and J level. Although the isotopic dependence of the linewidths was observed in the fundamental, no perceptible J-dependence was detected. The observed rotational temperature of $N_{2}HF, 17K,$ is higher than the typical rotational temperatures observed for complexes produced in argon expansion of 11 K