A Survey of Open Clusters in the u'g'r'i'z' Filter System: I. Results for NGC2548 (M48)

We present initial results of a photometric survey of open star clusters, primarily in the southern hemisphere, taken in the u'g'r'i'z' filter system. While our entire observed sample covers more than 100 clusters, here we present data for NGC2548 (M48) which is a cluster characterized in the UBV and DDO photometric systems. We compare our results to the published values from other observers and to the Padova theoretical isochrones and metallicity curves. These observations demonstrate that the u'g'r'i'z' filters can play an important role in determining the metallicity of stars and clusters. We begin this series of papers with a study of NGC2548 because we have obtained data of this cluster not only with our main program telescope, the CTIO Curtis-Schmidt, but also with the US Naval Observatory (USNO) 1.0m telescope (the telescope used to define the u'g'r'i'z' system), and the Sloan Digital Sky Survey (SDSS) 0.5m Photometric Telescope (the photometric monitoring telescope used to calibrate the SDSS 2.5m telescope imaging data). We have used the data from this study to validate our ability to transform measurements obtained on other telescopes to the standard USNO 1.0m u'g'r'i'z' system. This validation is particularly important for very red stars, for which the original u'g'r'i'z' standard star network is poorly constrained.Comment: 32 pages, 8 figures. Complete, machine-readable versions of Tables 4-6 available at http://home.fnal.gov/~dtucker/OpenClusters/NGC2548/ . Accepted for publication in the Astronomical Journa

Efflux of compatible solutes in Corynebacterium glutamicum mediated by osmoregulated channel activity

Ruffert S, Lambert C, Peter H, Wendisch VF, Kramer R. Efflux of compatible solutes in Corynebacterium glutamicum mediated by osmoregulated channel activity. European Journal of Biochemistry. 1997;247(2):572-580.Bacteria respond to hypoosmotic stress by releasing low-molecular-mass solutes in older to maintain constant turgor pressure. We have studied the function of osmoregulated channel(s) in Corynebactium glutamicum which are responsible for efflux of various solutes upon sudden decrease in osmotic pressure. The channels preferentially mediated efflux of compatible solutes such as glycine betaine and proline. The release of molecules of similar size. e.g. glutamate or lysine, was restricted, ATP was completely retained even after severe osmotic shock. The cells maintained high cytoplasmic K+ and Na+ concentrations under hypoosmotic shock. Several results suggest that the solute efflux is mediated by a channel and not by a carrier. e.g. by reversal of the glycine betaine uptake systems of C. glutamicum,lf the release of glycine betaine and proline was extremely fast reaching an efflux rate of 6000 mu mol . min(-1). g dm(-1) or higher: the efflux was not significantly influenced by addition of external transport substrate, e.g. glycine betaine; in spite of an extremely high chemical gradient, no significant efflux under isoosmolar conditions was observed: efflux of solutes was unchanged after full uncoupling of membrane energetics by carbonylcyanide m-chlorophenylhydrazone (CCCP). These results indicate the presence of an osmoregulated channel in C. glutamicum similar to the mechanosensitive channel(s) of Escherichia coli. The activity of the channel did not depend on the growth conditions, but we observed a light regulation on the level of activity, i.e. the mechanosensitive channel behaved as a perfect osmometer. By monitoring release of glycine betaine under slow and continuous decrease of the external osmolality, we observed continous efflux without a stepwise release of solutes. This resulted in a significant steady-state decrease ase of the membrane potential