Synthesis of glycine betaine from exogenous choline in the moderately halophilic bacterium halomonas elongata

Abstract

The role of choline in osmoprotection in the moderate halophile Halomonas elongata has been examined. Transport and conversion of choline to betaine began immediately after addition of choline to the growth medium. Intracellular accumulation of betaine synthesized from choline was salt dependent up to 2.5 M NaCl. Oxidation of choline was enhanced at 2.0 M NaCl in the presence or absence of externally provided betaine. This indicates that the NaCl concentration in the growth medium has major effects on the choline-betaine pathway of H. elongata.

FIG. 1

Effect of choline on growth of H. elongata in M63-glucose-minimal medium with increasing salinity. Cultures were grown at 37°C in M63 containing the indicated concentrations of NaCl, and the optical density was determined after 24 h. Symbols: •, M63 alone; ○, M63 with 1 mM choline. For every growth experiment, data are the averages of three different repeats which on no occasion varied by more than 5%. OD₆₀₀, optical density at 600 nm.

FIG. 2

Analysis of the conversion of choline to betaine in response to salinity. Choline and betaine in cell extracts were separated by thin-layer chromatography and visualized by autoradiography. Signals corresponding to choline (closed symbols) and betaine (open symbols) in the autoradiogram were analyzed by computing densitometry. Cells were grown at 0.5 (circles) or 2.0 (squares) M NaCl. Experiments (at both salt concentrations) are means of duplicates which on no occasion varied by more than 5%.

FIG. 3

Effect of betaine on the intracellular oxidation of choline. H. elongata was grown in M63 containing 0.5 (circles) or 2.0 (squares) M NaCl and [methyl-¹⁴C]choline (closed symbols) and betaine (open symbols). Oxidation of choline was determined as described in the legend to Fig. 2. Experiments (at both salt concentrations) were repeated twice, and the data correspond to means which on no occasion varied by more than 5%.

FIG. 4

Accumulation of betaine from choline by H. elongata in M63 at increasing osmolarities. Cells were grown overnight in the presence of [methyl-¹⁴C]choline and [³H]glucose. Samples were centrifuged and washed, and the accumulated radioactivity was measured by scintillation counting. Experiments (at both salt concentrations) were repeated twice, and the data correspond to means which on no occasion varied by more than 5%. conc, concentration.