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Different Spectra of Stationary-Phase Mutations in Early-Arising versus Late-Arising Mutants of Pseudomonas putida: Involvement of the DNA Repair Enzyme MutY and the Stationary-Phase Sigma Factor RpoS. Signe Saumaa, 2002.Stationary-phase mutations occur in populations of stressed, nongrowing, and slowly growing cells and allow mutant bacteria to overcome growth barriers . Mutational processes in starving cells are different from those occurring in growing bacteria . Here, we present evidence that changes in mutational processes also take place during starvation of bacteria . Our test system for selection of mutants based on creation of functional promoters for the transcriptional activation of the phenol degradation genes pheBA in starving Pseudomonas putida enables us to study base substitutions (C-to-A or G-to-T transversions), deletions, and insertions . We observed changes in the spectrum of promoter-creating mutations during prolonged starvation of Pseudomonas putida on phenol minimal plates . One particular C-to-A transversion was the prevailing mutation in starving cells . However, with increasing time of starvation, the importance of this mutation decreased but the percentage of other types of mutations, such as 2- to 3-bp deletions, increased . The rate of transversions was markedly elevated in the P . putida MutY-defective strain . The occurrence of 2- to 3-bp deletions required the stationary-phase sigma factor RpoS, which indicates that some mutagenic pathway is positively controlled by RpoS in P . putida . YjdE (AdiC) Is the Arginine:Agmatine Antiporter Essential for Arginine-Dependent Acid Resistance in Escherichia coli. Shimei Gong, 2003.To survive in extremely acidic conditions, Escherichia coli has evolved three adaptive acid resistance strategies thought to maintain internal pH . While the mechanism behind acid resistance system 1 remains enigmatic, systems 2 and 3 are known to require external glutamate (system 2) and arginine (system 3) to function . These latter systems employ specific amino acid decarboxylases and putative antiporters that exchange the extracellular amino acid substrate for the intracellular by-product of decarboxylation . Although GadC is the predicted antiporter for system 2, the antiporter specific for arginine/agmatine exchange has not been identified . A computer-based homology search revealed that the yjdE (now called adiC) gene product shared an overall amino acid identity of 22% with GadC . A series of adiC mutants isolated by random mutagenesis and by targeted deletion were shown to be defective in arginine-dependent acid resistance . This defect was restored upon introduction of an adiC+-containing plasmid . An adiC mutant proved incapable of exchanging extracellular arginine for intracellular agmatine but maintained wild-type levels of arginine decarboxylase protein and activity . Western blot analysis indicated AdiC is an integral membrane protein . These data indicate that the arginine-to-agmatine conversion defect of adiC mutants was at the level of transport . The adi gene region was shown to be organized into two transcriptional units, adiAY and adiC, which are coordinately regulated but independently transcribed . The data also illustrate that the AdiA decarboxylase:AdiC antiporter system is designed to function only at acid levels sufficient to harm the cell . Identification and Manipulation of Soil Properties To Improve the Biological Control Performance of Phenazine-Producing Pseudomonas fluorescens. Bonnie H. Ownley, 2003.Pseudomonas fluorescens 2-79RN10 protects wheat against take-all disease caused by Gaeumannomyces graminis var . tritici; however, the level of protection in the field varies from site to site . Identification of soil factors that exert the greatest influence on disease suppression is essential to improving biocontrol . In order to assess the relative importance of 28 soil properties on take-all suppression, seeds were treated with strain 2-79RN10 (which produces phenazine-1-carboxylate [PCA+]) or a series of mutants with PCA+ and PCA- phenotypes . Bacterized seeds were planted in 10 soils, representative of the wheat-growing region in the Pacific Northwest . Sixteen soil properties were correlated with disease suppression . Biocontrol activity of PCA+ strains was positively correlated with ammonium-nitrogen, percent sand, soil pH, sodium (extractable and soluble), sulfate-sulfur, and zinc . In contrast, biocontrol was negatively correlated with cation-exchange capacity (CEC), exchangeable acidity, iron, manganese, percent clay, percent organic matter (OM), percent silt, total carbon, and total nitrogen . Principal component factor analysis of the 16 soil properties identified a three-component solution that accounted for 87 percent of the variance in disease rating (biocontrol) . A model was identified with step-wise regression analysis (R2 = 0.96; Cp statistic = 6.17) that included six key soil properties: ammonium-nitrogen, CEC, iron, percent silt, soil pH, and zinc . As predicted by our regression model, the biocontrol activity of 2-79RN10 was improved by amending a soil low in Zn with 50 µg of zinc-EDTA/g of soil . We then investigated the negative correlation of OM with disease suppression and found that addition of OM (as wheat straw) at rates typical of high-OM soils significantly reduced biocontrol activity of 2-79RN10 .
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