Glycine betaine synthesis and its role for salt tolerance in a halotolerant cyanobacterium Aphanothece halophytica

Abstract

Glycine betaine functions as an efficient osmoprotectant for Aphanothece halophytica in high-osmolarity environments. This cell can accumulate glycine betaine through de novo synthesis in defined medium. By radiotracer experiments, it was found that A. halophytica could synthesize glycine betaine, using choline as a precursor, via an intermediate betaine aldehyde. Other precursors, namely ethanolamine and glycine could also be converted to glycine betaine. Morever, the formation of glycine betaine from glycine occurred at a faster rate than that from choline or ethanolamine. Under salt stress, A. halophytica exhibited an increased glycine betaine biosynthesis using choline or ethanolamine or glycine as a precursor. The level of intracellular glycine betaine also increased under salt stress. The activity of choline dehydrogenase, the first enzyme in choline-glycine betaine pathway, was found to localize mainly in the membrane fraction of the disrupte cells. The enzyme activity was enhanced by the hypersalinity of the growth medium.