Physiological role of compatible solute hydroxyectoine, synthesized in the halophilic Halomonas ventosae DL7

Halomonas ventosae DL7 is a moderately halophilic bacterium that can grow over a wide range of salinity, 1-23 % NaCl. In response to the external osmotic stress (1-3 M NaCl), strain DL7 can accumulate ectoine and hydroxyectoine as compatible solutes, both of which were identified by nuclear magnetic resonance (NMR) spectroscopy. Hydroxyectoine, derivative of ectoine, was synthesized as minor compound (10.1 mg/L) when incubated in nutrient medium at 30degC. However, the concentration of hydroxyectoine significantly increased with the increase of incubation temperature, the amount of which reached 81.8 mg /L at 42degC. Hydroxyectoine was converted from ectoine by hydroxylase and its gene fragments have been obtained by PCR amplification. In the mutant Escherichia coli strain which harbors the ectD gene, hydroxylase has been successfully expressed at 42degC, in which conversion from ectoine to hydroxyectoine was observed, but not as high as expected. These data suggest that hydroxyectoine was synthesized by complex reactions in H. ventosae DL7 to cope with the alteration of external environments.