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“Serine hydroxymethyltransferase

(SHMT) is a key enzyme in cellular one-carbon pathway and has been studied in many living organisms from bacteria to higher plants and mammals. However, biochemical and molecular characterization of SHMT from photoautotrophic microorganisms remains a challenge. Here, we isolated the SHMT gene from a halotolerant cyanobacterium Aphanothece halophytica (ApSHMT) and expressed it in Escherichia coli. Purified recombinant ApSHMT protein exhibited catalytic reactions for dl-threo-3-phenylserine as well as for l-serine. Catalytic reaction for l-serine was strongly inhibited by NaCl, but not to that level with glycine betaine. Overexpression of ApSHMT in E. coli resulted in the increased accumulation of glycine and serine. Choline and glycine betaine

levels were also significantly Venetoclax find more increased. Under high salinity, the growth rate of ApSHMT-expressing cells was faster compared to its respective control. High salinity also strongly induced the transcript level of ApSHMT in A. halophytica. Our results indicate the importance of a novel pathway; salt-induced ApSHMT increased the level of glycine betaine via serine and choline and conferred the tolerance to salinity stress. Serine is an essential amino acid, and that plays important roles in a variety of biological processes including metabolism, purine and pyrimidine biosynthesis, and generation of activated one-carbon (C-1) unit

(Beaudin et al., 2011). Through serine hydroxymethyltransferase (SHMT), serine associates with glycine metabolism via the glycine decarboxylase complex (GDC). SHMT is a pyridoxal 5′-phosphate (PLP)-dependent many enzyme catalyzing the interconversion of serine and tetrahydrofolate (THF) to glycine and N5, N10-methylene-THF (Schirch et al., 1985). In mammals, SHMT has been shown to be involved in de novo biosynthesis of thymidylate (Anderson & Stover, 2009). Disruption of SHMT increases the risk of neural tube defects (Anderson & Stover, 2009; Beaudin et al., 2011). In prokaryotes such as Escherichia coli, 15% of all carbon atoms assimilated from glucose is estimated to pass through the glycine–serine pathway (Wilson et al., 1993). In plants, SHMT cooperates with the GDC to mediate photorespiratory glycine–serine interconversion (Voll et al., 2005; Bauwe et al., 2010). In cyanobacteria, the SHMT gene was suggested to be essential for cell survival because the complete segregation of SHMT gene could not be generated (Hagemann et al., 2005). Although the enzyme activity of SHMT from a cyanobacterium Synechocystis sp. PCC 6803 has been determined (Eisenhut et al., 2006), molecular properties of cyanobacterial SHMT remain largely unknown. Here, we report on the molecular and biochemical characterization of a putative ApSHMT gene from a halotolerant cyanobacterium Aphanothece halophytica (hereafter called A.