Genes for the synthesis of the osmoprotectant glycine betaine from choline in the moderately halophilic bacterium Halomonas elongata DSM 3043, USA.

The genes involved in the oxidative pathway of choline to glycine betaine in the moderate halophile Halomonas elongata DSM 3043 were isolated by functional complementation of an Escherichia coli strain defective in glycine betaine synthesis. The cloned region was able to mediate the oxidation of choline to glycine betaine in E. coli, but not the transport of choline, indicating that the gene(s) involved in choline transport are not clustered with the glycine betaine synthesis genes. Nucleotide sequence analysis of a 4.6 kb segment from the cloned DNA revealed the occurrence of three ORFs (betIBA) apparently arranged in an operon. The deduced betI gene product exhibited features typical for DNA-binding regulatory proteins. The deduced BetB and BetA proteins showed significant similarity to soluble glycine betaine aldehyde dehydrogenases and membrane-bound choline dehydrogenases, respectively, from a variety of organisms. Evidence is presented that BetA is able to oxidize both choline and glycine betaine aldehyde and therefore can mediate both steps in the synthesis of glycine betaine.