7,668 research outputs found
Parsec-scale structure of quasars: dawn of the golden age?
Half a century after their discovery, the study of quasars remains one of the
most fascinating intellectual challenges in astronomy. Quasars are laboratories
for everything from relativity to magnetohydrodynamics and are perhaps the best
available probes for cosmology. A tremendous amount has been learned about
quasars and yet many of the most fundamental questions about their physics
remain open. Parsec-scale observations have played an indispensable role in
building up our current understanding of quasars; virtually everything we know
about quasars depends on such observations. However, the finest hour for parsec
scale observations may be just beginning. This is partly due to the development
of highly reliable VLBI networks (which is continuing) but mostly due to the
unprecedented availability of multiepoch, simultaneous, broadband observations
that have long been the `holy grail' for quasar researchers.Comment: Accepted for publication in the Bulletin of the Astronomical Society
of India (20 pages, 3 figures
The role of iron in Mycobacterium smegmatis biofilm formation: The exochelin siderophore is essential in limiting iron conditions for biofilm formation but not for planktonic growth
Many species of mycobacteria form structured biofilm communities at liquid-air interfaces and on solid surfaces. Full development of Mycobacterium smegmatis biofilms requires addition of supplemental iron above 1 Ī¼M ferrous sulphate, although addition of iron is not needed for planktonic growth. Microarray analysis of the M. smegmatis transcriptome shows that iron-responsive genes - especially those involved in siderophore synthesis and iron uptake - are strongly induced during biofilm formation reflecting a response to iron deprivation, even when 2 Ī¼M iron is present. The acquisition of iron under these conditions is specifically dependent on the exochelin synthesis and uptake pathways, and the strong defect of an iron-exochelin uptake mutant suggests a regulatory role of iron in the transition to biofilm growth. In contrast, although the expression of mycobactin and iron ABC transport operons is highly upregulated during biofilm formation, mutants in these systems form normal biofilms in low-iron (2 Ī¼M) conditions. A close correlation between iron availability and matrix-associated fatty acids implies a possible metabolic role in the late stages of biofilm maturation, in addition to the early regulatory role. M. smegmatis surface motility is similarly dependent on iron availability, requiring both supplemental iron and the exochelin pathway to acquire it. Ā© 2007 The Authors
- ā¦