42 research outputs found
Evolutionary analysis implicates RNA polymerase II pausing and chromatin structure in nematode piRNA biogenesis
Piwi-interacting RNAs (piRNAs) control transposable elements widely across metazoans but have rapidly evolving biogenesis pathways. In Caenorhabditis elegans, almost all piRNA loci are found within two 3Mb clusters on Chromosome IV. Each piRNA locus possesses an upstream motif that recruits RNA polymerase II to produce a ~28 nt precursor transcript. Here, we use comparative epigenomics across nematodes to gain insight into piRNA biogenesis. We show that the piRNA upstream motif is derived from core promoter elements controlling snRNA biogenesis. We describe two alternative modes of piRNA organisation in nematodes: in C. elegans and closely related nematodes, piRNAs are clustered within repressive H3K27me3 chromatin, whilst in other species, typified by Pristionchus pacificus, piRNAs are distributed genome-wide within introns of actively transcribed genes. In both groups, piRNA production depends on downstream sequence signals associated with RNA polymerase II pausing, which synergise with the chromatin environment to control piRNA precursor transcription
Analysis of the Interaction Effects in the Southern Galaxy Pair Tol1238-364 and ESO381-G009
In the context of the connection among galaxy-galaxy interaction, starbursts
and nuclear activity, we present and discuss a quantitative morphological
analysis based on BVR images and a detailed spectroscopic investigation of two
interacting galaxies, the Seyfert 2 Tol1238-364 (IC 3639) and its companion
ESO381-G009, forming a triple system with ESO381-G006. Broad-band optical
photometry is complemented by Halpha imaging, which provides information about
the distribution of star forming regions across the galaxies. Long-slit
spectroscopic data obtained at different position angles of the slit are
employed to determine the physical conditions of circumnuclear and extranuclear
regions. A mixture of thermal and non-thermal ionizing radiation is found in
the surroundings of the nucleus of Tol1238-364 and the energy budget supports
the presence of a circumnuclear starburst.Several regions in both the galaxies
show anomalous line ratios: additional ionization by shock-heating and low
ionization of some extranuclear HII regions are suggested as possible
explanations. An analysis of the emission-line profiles reveals the presence of
a broad Halpha component in the nuclear region of Tol1238-364. Independent
estimates of the star formation rates (SFR) were obtained through
flux-calibrated Halpha-images and FIR emission in the four IRAS bands. Overall
SFR densities have been compared with the SFR densities derived from Halpha
emission in the individual regions of the galaxies sampled by long-slit
spectra. In both galaxies an enhancement of the star formation activity with
respect to isolated galaxies is revealed. The prevalence of starburst or
nuclear activity has been examined through FIR color indices. The interaction
scenario is discussed on the basis of the observed galaxy properties.Comment: Tentatively scheduled for publication in ApJS, v184 n2 October 2003
issue. A version with full resolution figures is available as unedited
preprint at http://www.journals.uchicago.edu/ApJ/future.htm
Comparative Epigenomics Reveals that RNA Polymerase II Pausing and Chromatin Domain Organization Control Nematode piRNA Biogenesis.
Piwi-interacting RNAs (piRNAs) are important for genome regulation across metazoans, but their biogenesis evolves rapidly. In Caenorhabditis elegans, piRNA loci are clustered within two 3-Mb regions on chromosome IV. Each piRNA locus possesses an upstream motif that recruits RNA polymerase II to produce an âŒ28 nt primary transcript. We used comparative epigenomics across nematodes to gain insight into the origin, evolution, and mechanism of nematode piRNA biogenesis. We show that the piRNA upstream motif is derived from core promoter elements controlling snRNA transcription. We describe two alternative modes of piRNA organization in nematodes: in C. elegans and closely related nematodes, piRNAs are clustered within repressive H3K27me3 chromatin, while in other species, typified by Pristionchus pacificus, piRNAs are found within introns of active genes. Additionally, we discover that piRNA production depends on sequence signals associated with RNA polymerase II pausing. We show that pausing signals synergize with chromatin to control piRNA transcription.Work in the Sarkies laboratory is funded by a grant from the Medical Research Council MC-A652-5PY80. P.S. was funded by an Imperial College Research Fellowship. L.S. was funded by a Bailie-Gifford PhD studentship. We thank the London Institute of Medical Sciences Genomics Facility for sequencing. Some sequencing was carried out at Edinburgh Genomics, which has core support from the NERC Biomolecular Analysis Facility award UKSBS PR18037. Work in the MartĂnez-PĂ©rez laboratory was funded by a grant from the Medical Research Council MC-A652-5PY60. G.S. was funded by a Newton International Fellowship (Royal Society). J.A. was funded by a Wellcome Senior Research Fellowship (101863). T.Y.B. was funded by a Genetics Society Summer Studentship to the Sarkies lab
A long-term photometric study of V 1184 Tauri
Aims. The main purpose of our investigation is to construct the long-time light curve of the PMS star V 1184 Tau. We consider the study of the photometrical variability of PMS stars as very important for understanding stellar evolution. The unusual photometric variability of V 1184 Tau was reported in previous studies as well, but the nature of the observed deep minima is still under discussion. Methods: We present recent data from CCD photometry and from archival photographic plates. The photometric BVRI data presented in this paper were collected from November 2005 to November 2007. To construct the historical light curve of V 1184 Tau, a search for archived photographic observations in the Wide-Field Plate Database (WFPDB) was made. As a result, 412 photographic plates were found containing the field of V 1184 Tau. A part of the plates were scanned at our request and a magnitude estimation was made of V 1184 Tau. Results: Our recent photometric data suggest that the star brightness is still near the minimum. Thus the period of strong light variations that started in 2003 continues up to the present, hence more than 4 years. Our data from the archival photographic plates suggest that an unknown minimum of brightness exists in the approximate period 1980-1985. Taking all available photometric and spectroscopic data into account we must reject the hypothesis that V 1184 Tau is an FU Orionis type object. V 1184 Tau is a G type low-mass star whose spectrum is similar to WTT stars, but its photometric behavior is typical of the UX Orionis variable stars. Assuming the obscuration from orbiting dust clouds as a reason for a deep minimum, we estimate the approximate period of obscurations and the interval between two deep minima. Our calculations give a 25-28 year period between the two minima and approximately 8-10 year duration of the minima