Abstract
Glycine betaine is an important osmoprotectant and synthesized in response to abiotic stresses. Although almost all known biosynthetic pathways of betaine are two-step oxidation of choline, we isolated two N-methyltransferase genes from a halotolerant cyanobacterium Aphanothece halophytica. One of gene products catalyzed the methylation reactions of glycine and sarcosine with S-adenosylmethionine acting as the methyl donor. The other one specifically catalyzed the methylation of dimethylglycine to betaine. Both enzymes are active as monomers. Betaine did not show the feed back inhibition for the methyltransferases even in the presence of 2 M. S-Adenosyl homocysteine inhibited the methylation reactions with relatively low affinities. The co-expressing of two enzymes in E. coli increased the betaine level and enhanced the growth rates. Immunoblot analysis revealed that the accumulation levels of both enzymes increased with increasing the salinity. Physiological and functional properties of methyltransferases were discussed.
