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Regulatory RNA as Mediator in GacA/RsmA-Dependent Global Control of Exoproduct Formation in Pseudomonas fluorescens CHA0. Stephan Heeb, 2002.In Pseudomonas fluorescens CHA0, an antagonist of root-pathogenic fungi, the GacS/GacA two-component system tightly controls the expression of antifungal secondary metabolites and exoenzymes at a posttranscriptional level, involving the RNA-binding protein and global regulator of secondary metabolism RsmA . This protein was purified from P . fluorescens, and RNA bound to it was converted to cDNA, which served as a probe to isolate the corresponding chromosomal locus, rsmZ . This gene encoded a regulatory RNA of 127 nucleotides and a truncated form lacking 35 nucleotides at the 3" end . Expression of rsmZ depended on GacA, increased with increasing population density, and was stimulated by the addition of a solvent-extractable extracellular signal produced by strain CHA0 at the end of exponential growth . This signal appeared to be unrelated to N-acyl-homoserine lactones . A conserved upstream element in the rsmZ promoter, but not the stress sigma factor RpoS, was involved in rsmZ expression . Overexpression of rsmZ effectively suppressed the negative effect of gacS and gacA mutations on target genes, i.e., hcnA (for hydrogen cyanide synthase) and aprA (for the major exoprotease) . Mutational inactivation of rsmZ resulted in reduced expression of these target genes in the presence of added signal . Overexpression of rsmA had a similar, albeit stronger negative effect . These results support a model in which GacA upregulates the expression of regulatory RNAs, such as RsmZ of strain CHA0, in response to a bacterial signal . By a titration effect, RsmZ may then alleviate the repressing activity of RsmA on the expression of target mRNAs . Heterotrophic Bacterial Growth Efficiency and Community Structure at Different Natural Organic Carbon Concentrations. Alexander Eiler, 2003.Batch cultures of aquatic bacteria and dissolved organic matter were used to examine the impact of carbon source concentration on bacterial growth, biomass, growth efficiency, and community composition . An aged concentrate of dissolved organic matter from a humic lake was diluted with organic compound-free artificial lake water to obtain concentrations of dissolved organic carbon (DOC) ranging from 0.04 to 2.53 mM . The bacterial biomass produced in the cultures increased linearly with the DOC concentration, indicating that bacterial biomass production was limited by the supply of carbon . The bacterial growth rate in the exponential growth phase exhibited a hyperbolic response to the DOC concentration, suggesting that the maximum growth rate was constrained by the substrate concentration at low DOC concentrations . Likewise, the bacterial growth efficiency calculated from the production of biomass and CO2 increased asymptotically from 0.4 to 10.4% with increasing DOC concentration . The compositions of the microbial communities that emerged in the cultures were assessed by separation of PCR-amplified 16S rRNA fragments by denaturing gradient gel electrophoresis . Nonmetric multidimensional scaling of the gel profiles showed that there was a gradual change in the community composition along the DOC gradient; members of the ß subclass of the class Proteobacteria and members of the Cytophaga-Flavobacterium group were well represented at all concentrations, whereas members of the  subclass of the Proteobacteria were found exclusively at the lowest carbon concentration . The shift in community composition along the DOC gradient was similar to the patterns of growth efficiency and growth rate . The results suggest that the bacterial growth efficiencies, the rates of bacterial growth, and the compositions of bacterial communities are not constrained by substrate concentrations in most natural waters, with the possible exception of the most oligotrophic environments .
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