Construction and Characterization of a proU - gfp Transcriptional Fusion That Measures Water Availability in a Microbial Habitat

Abstract

ABSTRACT We constructed and characterized a transcriptional fusion that measures the availability of water to a bacterial cell. This fusion between the proU promoter from Escherichia coli and the reporter gene gfp was introduced into strains of E. coli , Pantoea agglomerans , and Pseudomonas syringae . The proU - gfp fusion in these bacterial biosensor strains responded in a quantitative manner to water deprivation caused by the presence of NaCl, Na 2 SO 4 , KCl, or polyethylene glycol (molecular weight, 8000). The fusion was induced to a detectable level by NaCl concentrations of as low as 10 mM in all three bacterial species. Water deprivation induced proU - gfp expression in both planktonic and surface-associated cells; however, it induced a higher level of expression in the surface-associated cells. Following the introduction of P. agglomerans biosensor cells onto bean leaves, the cells detected a significant decrease in water availability within only 5 min. After 30 min, the populations were exposed, on average, to a water potential equivalent to that imposed by approximately 55 mM NaCl. These results demonstrate the effectiveness of a proU - gfp -based biosensor for evaluating water availability on leaves. Furthermore, the inducibility of proU - gfp in multiple bacterial species illustrates the potential for tailoring proU - gfp -based biosensors to specific habitats.