21 research outputs found
Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters
Testing Newtonian gravity in the weak-acceleration regime is vital to our
understanding of the nature of the gravitational interaction. It has recently
been claimed that the velocity dispersion profiles of several globular clusters
flatten out at large radii, reminiscent of galaxy rotation curves, even though
globular clusters are thought to contain little or no dark matter. We
investigate this claim, using AAOmega observations of four globular clusters,
namely M22, M30, M53 and M68. M30, one such cluster that has had this claim
made for its velocity dispersion, was included for comparison with previous
studies. We find no statistically significant flattening of the velocity
dispersion at large radii for any of our target clusters and therefore we infer
the observed dynamics do not require that globular clusters are dark matter
dominated, or a modification of gravity. Furthermore, by applying a simple
dynamical model we determine the radial mass-to-light profiles for each
cluster. The isothermal rotations of each cluster are also measured, with M22
exhibiting clear rotation, M68 possible rotation and M30 and M53 lacking any
rotation, within the uncertainties.Comment: 7 pages, 4 figures and two tables. Accepted by MNRA
MRI findings in central nervous system systemic lupus erythematosus are associated with immunoserological parameters and hypertension
Integrated pararetroviral sequences define a unique class of dispersed repetitive DNA in plants
Small nucleolar RNA interference induced by antisense or double-stranded RNA in trypanosomatids
In trypanosomes the C/D- and H/ACA-like small nucleolar RNAs (snoRNAs) are clustered and repeated in the genome. The snoRNAs studied to date are transcribed as polycistronic transcripts by RNA polymerase II and then processed, resulting in mature snoRNAs. In this study we demonstrated that snoRNA genes can be silenced in three trypanosomatid species: Leptomonas collosoma, Leishmania major, and Trypanosoma brucei. Silencing was achieved in L. collosoma and L. major by the expressing of an antisense transcript complementary to the snoRNA gene and was accompanied by the accumulation of small interfering RNA. Silencing eliminated the mature snoRNA but not its precursor and abolished the specific 2′-O-methylation guided by the snoRNA. In T. brucei, silencing was achieved by using the inducible synthesis of double-stranded RNA from T7 opposing promoters. Silencing varied between the different snoRNA genes, which may reflect the accessibility of small interfering RNA to the target RNAs. This study suggests that RNA interference can degrade snoRNAs. This study has further implications in elucidating the function of nucleolar RNAs and specific modifications guided by these RNAs in trypanosomatids and perhaps in other eukaryotes as well
The SNORD115 (H/MBII-52) and SNORD116 (H/MBII-85) gene clusters at the imprinted Prader-Willi locus generate canonical box C/D snoRNAs
Position within the host intron is critical for efficient processing of box C/D snoRNAs in mammalian cells
In mammalian cells, all small nucleolar RNAs (snoRNAs) that guide rRNA modification are encoded within the introns of host genes. A database analysis of human box C/D snoRNAs revealed conservation of their intronic location, with a preference for 70–80 nt upstream of the 3′ splice site. Transfection experiments showed that synthesis of gas5-encoded U75 and U76 snoRNAs dropped significantly for mutant constructs possessing longer or shorter spacers between the snoRNA and the 3′ splice site. However, the position of the snoRNA did not affect splicing of the host intron. Substitution mutations within the spacer indicated that the length, but not the specific sequence, is important. A in vitro system that couples pre-mRNA splicing and processing of U75 has been developed. U75 synthesis in vitro depends on its box C and D sequences and requires an appropriate spacer length. Further mutational analyses both in vivo and in vitro, with subsequent mapping of the branch points, revealed that the critical distance is from the snoRNA coding region to the branch point, suggesting synergy between splicing and snoRNA release