49 research outputs found
Mining microbial genomes for new natural products and biosynthetic pathways
Analyses of microbial genome sequences have revealed numerous examples of âcrypticâ or âorphanâ biosynthetic gene clusters, with the potential to direct the production of novel, structurally complex natural products. This article summarizes the various methods that have been developed for discovering the products of cryptic biosynthetic gene clusters in microbes and gives an account of my group's discovery of the products of two such gene clusters in the model actinomycete Streptomyces coelicolor M145. These discoveries hint at new mechanisms, roles and specificities for natural product biosynthetic enzymes. Our efforts to elucidate these are described. The identification of new secondary metabolites of S. coelicolor raises the question: what is their biological function? Progress towards answering this question is also summarized
3D Integral Field Observations of Ten Galactic Winds - I. Extended phase (>10 Myr) of mass/energy injection before the wind blows
We present 3D spectroscopic observations of a sample of 10 nearby galaxies
with the AAOmega-SPIRAL integral field spectrograph on the 3.9m AAT, the
largest survey of its kind to date. The double-beam spectrograph provides
spatial maps in a range of spectral diagnostics: [OIII] 5007, H-beta, Mg-b,
NaD, [OI] 6300, H-alpha, [NII] 6583, [SII] 6717, 6731. All of the objects in
our survey show extensive wind-driven filamentation along the minor axis, in
addition to large-scale disk rotation. Our sample can be divided into either
starburst galaxies or active galactic nuclei (AGN), although some objects
appear to be a combination of these. The total ionizing photon budget available
to both classes of galaxies is sufficient to ionise all of the wind-blown
filamentation out to large radius. We find however that while AGN
photoionisation always dominates in the wind filaments, this is not the case in
starburst galaxies where shock ionisation dominates. This clearly indicates
that after the onset of star formation, there is a substantial delay (> 10 Myr)
before a starburst wind develops. We show why this behavior is expected by
deriving ``ionisation'' and dynamical timescales for both AGNs and starbursts.
We establish a sequence of events that lead to the onset of a galactic wind.
The clear signature provided by the ionisation timescale is arguably the
strongest evidence yet that the starburst phenomenon is an impulsive event. A
well-defined ionisation timescale is not expected in galaxies with a protracted
history of circumnuclear star formation. Our 3D data provide important
templates for comparisons with high redshift galaxies.[Abridged]Comment: 43 pages, 30 figures, Accepted for publication in ApJ Jan-2010, Full
resolution figures available from:
http://www.aao.gov.au/AAO/local/www/rgs/work/winds/public/papers/SPIRAL_WINDS_hi-res.htm
GMASS Ultradeep Spectroscopy of Galaxies at redshift z~2. I. The stellar metallicity
Context: Galaxy metallicities have been measured to redshift z~2 by gas-phase
oxygen abundances of the interstellar medium using the R23 and N2 methods.
Galaxy stellar metallicities provide crucial data for chemical evolution models
but have not been assessed reliably much outside the local Universe. Aims: We
determine the iron-abundance, stellar metallicity of star-forming galaxies
(SFGs) at redshift z~2, observed as part of the Galaxy Mass Assembly ultra-deep
Spectroscopic Survey (GMASS). Methods: We compute the equivalent width of a
rest-frame mid-ultraviolet, photospheric absorption-line index, the 1978 index
found to vary monotonically with stellar metallicity by Rix and collaborators.
We normalise and combine 75 SFG spectra from the GMASS survey to produce a
spectrum corresponding to a total integration time 1652.5 hours (and a
signal-to-noise ratio ~100 for our 1.5 angstrom binning) of FORS2 spectroscopic
observations at the Very Large Telescope. Results: We measure an
iron-abundance, stellar metallicity of log (Z/Zsolar) = -0.574+/-0.159 for our
spectrum representative of a galaxy of stellar mass 9.4 x 10^9 Msolar assuming
a Chabrier IMF. We find that the R04 model SFG spectrum for log (Z/Zsolar) =
-0.699 solar metallicity provides the best description of our GMASS coadded
spectrum. For similar galaxy stellar mass, our stellar metallicity is ~0.25 dex
lower than the oxygen-abundance, gas-phase metallicity quantified by Erb and
collaborators for UV-selected star-forming galaxies at z=2. Conclusions: We
conclude that we are witnessing the establishment of a light-element
overabundance in galaxies as they are being formed at redshift z~2. Our
measurements are reminiscent of the alpha-element enhancement seen in
low-redshift, galactic bulges and early-type galaxies. (Abridged)Comment: Accepted for publication in Astronomy and Astrophysics on 18 December
2007, 9 pages, 8 figures, aa.bst and aa.cls A&A style file
Observations and modelling of a clumpy galaxy at z=1.6: Spectroscopic clues to the origin and evolution of chain galaxies
We investigate the properties of a clump-cluster galaxy at redshift 1.57. The
morphology of this galaxy is dominated by eight star-forming clumps in optical
observations, and has photometric properties typical of most clump-cluster and
chain galaxies. Its complex asymmetrical morphology has led to the suggestion
that this system is a group merger of several initially separate
proto-galaxies. We performed H_alpha integral field spectroscopy of this system
using SINFONI on VLT UT4. These observations reveal a large-scale velocity
gradient throughout the system, but with large local kinematic disturbances.
Using a numerical model of gas-rich disk fragmentation, we find that clump
interactions and migration can account for the observed disturbed rotation. On
the other hand, the global rotation would not be expected for a multiply
merging system. We further find that this system follows the stellar mass vs.
metallicity, star formation rate and size relations expected for a disk at this
redshift, and exhibits a disk-like radial metallicity gradient, so that the
scenario of internal disk fragmentation is the most likely one. A red and
metallic central concentration appears to be a bulge in this proto-spiral
clumpy galaxy. A chain galaxy at redshift 2.07 in the same field also shows
disk-like rotation. Such systems are likely progenitors of the present-day
bright spiral galaxies, forming their exponential disks through clump migration
and disruption and fueling their bulges. Our present results show that
disturbed morphologies and kinematics are not necessarily signs of galaxy
mergers and interactions, and can instead result from the internal evolution of
primordial disks.Comment: A&A, accepted. Version with full resolution figures available at
http://aramis.obspm.fr/~bournaud/udfr2.pdf -- Animation of clumpy galaxy
models available at http://aramis.obspm.fr/~bournaud/cc/cc.htm
Massive star formation in Wolf-Rayet galaxies. V: Star formation rates, masses and the importance of galaxy interactions
(Abridged) We have performed a comprehensive analysis of a sample of 20
starburst galaxies, most of them classified as Wolf-Rayet galaxies. In this
paper, the last of the series, we analyze the global properties of our galaxy
sample using multiwavelength data (X-ray, FUV, optical, NIR, FIR, and radio).
The agreement between our Ha-based SFR and those provided by indicators at
other wavelengths is remarkable, but we consider that the new Ha-based
calibration provided by Calzetti et al. (2007) should be preferred over older
calibrations. The FUV-based SFR provides a powerful tool to analyze the
star-formation activity in both global and local scales independently to the Ha
emission. We provide empirical relationships between the ionized gas mass,
neutral gas mass, dust mass, stellar mass, and dynamical mass with the
B-luminosity. Although all mass estimations increase with increasing
luminosity, we find important deviations to the general trend in some objects,
that seem to be consequence of their particular evolutionary histories. We
investigate the mass-metallicity relations and conclude that both the nature
and the star-formation history are needed to understand the relationships
between both properties. The majority of the galaxies follow a
Schmidt-Kennicutt scaling law of star-formation that agrees with that reported
in individual star-forming regions within M~51 but not with that found in
normal spiral galaxies. We found a relation between the reddening coefficient
and the warm dust mass indicating that the extinction is mainly internal to the
galaxies. Considering all data, we found that 17 up to 20 galaxies are clearly
interacting or merging with low-luminosity dwarf objects or HI clouds. We
conclude that interactions do play a fundamental role in the triggering
mechanism of the strong star-formation activity observed in dwarf starburst
galaxies.Comment: 33 pages, 21 figures, accepted for publication by A&
Brain/blood ratios of methadone and ABCB1 polymorphisms in methadone-related deaths
Methadone is an opioid that often leads to fatalities. Interpretation of toxicological findings can be challenging if no further information about the case history is available. The aims of this study were (1) to determine whether brain/blood ratios can assist in the interpretation of methadone findings in fatalities; (2) to examine whether polymorphisms in the gene encoding the P-glycoprotein (also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-family B member 1 (ABCB1)), which functions as a multispecific efflux pump in the blood-brain barrier, affect brain/blood ratios of methadone. Femoral venous blood and brain tissue (medulla oblongata and cerebellum) from 107 methadone-related deaths were analysed for methadone by gas chromatography-mass spectrometry. In addition, all the samples were genotyped for three common ABCB1 single nucleotide polymorphisms (SNPs rs1045642, rs1128503, and rs2032582) using ion-pair reversed-phase high-performance liquid chromatography-electrospray ionization mass spectrometry (ICEMS). In nearly all cases, methadone concentrations were higher in the brain than in the blood. Inter-individual brain/blood ratios varied (0.6-11.6); the mean ratio was 2.85 (standard deviation 1.83, median 2.35). Moreover, significant differences in mean brain/blood ratios were detected among the synonymous genotypes of rs1045642 in ABCB1 (p = 0.001). Cases with the T/T genotype had significantly higher brain/blood ratios than cases with the other genotypes (T/T vs. T/C difference (d) = 1.54, 95% CI [1.14, 2.05], p = 0.002; T/T vs. C/C d = 1.60, 95% CI [1.13, 2.29], p = 0.004). Our results suggest that the rs1045642 polymorphisms in ABCB1 may affect methadone concentrations in the brain and its site of action and may be an additional factor influencing methadone toxicity