An ALMA Dynamical Mass Estimate of the Proposed Planetary-mass Companion FW Tau C

Dynamical mass estimates down to the planet-mass regime can help to understand planet formation. We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm observations of FW Tau C, a proposed ~10 $M_{\rm Jup}$ planet-mass companion at ~330 au from the host binary FW Tau AB. We spatially and spectrally resolve the accretion disk of FW Tau C in ${}^{12}$CO (2-1). By modeling the Keplerian rotation of gas, we derive a dynamical mass of ~0.1 $M_\odot$. Therefore, FW Tau C is unlikely a planet, but rather a low-mass star with a highly inclined disk. This also suggests that FW Tau is a triple system consisting of three ~0.1 $M_\odot$ stars.Comment: Accepted for publication in ApJ

Submillimeter Array CO(2-1) Imaging of the NGC 6946 Giant Molecular Clouds

We present a CO(2-1) mosaic map of the spiral galaxy NGC 6946 by combining data from the Submillimeter Array and the IRAM 30 m telescope. We identify 390 giant molecular clouds (GMCs) from the nucleus to 4.5 kpc in the disk. GMCs in the inner 1 kpc are generally more luminous and turbulent, some of which have luminosities >10^6 K km/s pc^2 and velocity dispersions >10 km/s. Large-scale bar-driven dynamics likely regulate GMC properties in the nuclear region. Similar to the Milky Way and other disk galaxies, GMC mass function of NGC 6946 has a shallower slope (index>-2) in the inner region, and a steeper slope (index<-2) in the outer region. This difference in mass spectra may be indicative of different cloud formation pathways: gravitational instabilities might play a major role in the nuclear region, while cloud coalescence might be dominant in the outer disk. Finally, the NGC 6946 clouds are similar to those in M33 in terms of statistical properties, but they are generally less luminous and turbulent than the M51 clouds.Comment: Published in Ap

Length and temperature dependent crossover of charge transport across molecular junctions

We study the electronic transport in a molecular junction in which each site is coupled to a local phonon bath using the non-equilibrium Green's function method. We observe the length period of the oscillatory conductance in odd-numbered chains depends strongly on the applied bias, and the oscillatory behavior is smeared out for the bias voltage near the phonon energy. In addition, a crossover from tunneling to thermally activated hopping transport as the length of the molecule increases is found for the phonon-free case. In the presence of electron-phonon interaction, hopping transport is dominant and a transition from the thermally suppressed to assisted conduction is observed.Comment: 9 pages, 10 figure

HIV-1 Gene Expression: Transcriptional Regulation and RNA Interference Studies: a Dissertation

Gene expression of human immunodeficiency virus type-1 (HIV-1), which causes Acquired Immunodeficiency Syndrome (AIDS), is regulated at the transcriptional level, where negative factors can block elongation that is overcome by HIV Tat protein and P-TEFb. P-TEFb, a positive elongation transcription factor with two subunits, CDK9 and Cyclin T1 (CycT1), catalyzes Tat-dependent phosphorylation of Ser-5 in the Pol II C-terminal domain (CTD), allowing production of longer mRNAs. Ser-5 phosphorylation enables the CTD to recruit mammalian mRNA capping enzyme (Mce1) and stimulate its guanylyltransferase activity. This dissertation demonstrates that stable binding of Mce1 and cap methyltransferase to template-engaged Pol II depends on CTD phosphorylation, but not on nascent RNA. Capping and methylation doesn\u27t occur until nascent pre-mRNA become 19-22 nucleotides long. A second and novel pathway for recruiting and activating Mce1 involved direct physical interaction between the CTD, Tat and Mce1. Tat stimulated the guanylyltransferase and triphosphatase activities of Mce1, thereby enhancing the otherwise low efficiency of cotranscriptional capping of HIV mRNA. These findings imply that multiple mechanisms exist for coupling transcription elongation and mRNA processing at a checkpoint critical to HIV gene expression. To elucidate P-TEFb\u27s function in human (HeLa) cells, RNA interference (RNAi) was used to degrade mRNA for hCycT1 or CDK9. Down-regulation of P-TEFb expression by RNAi can be achieved without causing major toxic or lethal effects and can control Tat transactivation and HIV replication in host cells. High-density oligonucleotide arrays were used to determine the effect of P-TEFb knockdown on global gene expression. Of 44,928 human genes analyzed, 25 were down-regulated and known or likely to be involved in cell proliferation and differentiation. These results provide new insight into P-TEFb function, its potent role in early embryonic development and strong evidence that P-TEFb is a new target for developing AIDS and cancer therapies. To fulfill the promise of RNAi for treating infectious and human genetic diseases, structural and functional mechanisms underlying RNAi in human cells were studied. The status of the 5\u27 hydroxyl terminus of the antisense strand of short interfering RNA (siRNA) duplexes determined RNAi activity, while a 3\u27 terminus block was tolerated in vivo. A perfect A-form helix in siRNA was not required for RNAi, but was required for antisense-target RNA duplexes. Strikingly, crosslinking siRNA duplexes with psoralen did not completely block RNAi, indicating that complete unwinding of the siRNA helix is not necessary for RNAi in vivo. These results suggest that RNA amplification by RNA-dependent RNA polymerase is not essential for RNAi in human cells