SHMT, EC 2.1.2.1, catalyzes the interconversion of Ser and Gly accompanied by the production or consumption of one-carbon units. In leaves it is a component of the photo- respiratory pathway (Keys et al., 1978). The main function of that pathway is the salvage of reduced carbon and nitrogen withdrawn from the Calvin cycle by the oxygen- ation of ribulose 1,5-bisphosphate. By the cooFeration of SHMT with the mitochondrial Gly cleavage system two molecules of Gly are converted into Ser, C02, and NH,. This reaction is considered to be the exclusive source of photorespiratory CO, release (Somerville and Ogren, 1981). The enzyme was first isolated from rabbit liver (Schirch and Mason, 1963) and, since then, has been studied in a variety of organisms and tissues. From human, cDNAs of both the cytosolic and the mitochondrial iso-SHMT were cloned (Garrow et al., 1993). In addition, the cloning of SHMT