Thesis (M.Sc.)--Chulalongkorn University, 2011

Serine hydroxymethyltransferase (SHMT) is an important enzyme for serine-glycine cycles and plays important role as a central position in one-carbon metabolism. SHMT has been studied in many living organisms from bacteria to higher plants and mammals; however, biochemical and molecular characterization of SHMT has never been investigated in photoautotrophic microorganisms. In this study, we have isolated SHMT gene from a halotolerantcyanobacteriumAphanothecehalophytica (ApSHMT) and expressed in Escherichia coli. Recombinant ApSHMT exhibited catalytic reactions for both THF-dependent/independent cleavages. Catalytic reaction for L-serine, a physiological substrate, was strongly inhibited by NaCl but it could be protect efficiently by osmoprotectantbetaine. Amino acid analysis revealed the increased levels of amino acids (glycine and serine) in the expressing cells in all condition examined. Furthermore, metabolite analysis revealed alleviated levels of choline as well as betaine. ApSHMT expressing cells could accumulate betaine several folds higher than the control cells, which caused the enhanced growth rate of the expressing cells particularly under salt-stress conditions.