Characterization of three genes encoding enzymes of the folate biosynthetic pathway in Plasmodium falciparum

Although the folate metabolic pathway in malaria parasites is a major chemo therapeutic target, resistance to currently available antifolate drugs is a n increasing problem. This pathway, however, includes a number of enzymes t hat, to date, hale not been characterized despite their potential fur clini cal exploitation. As a step towards evaluation of additional targets in thi s pathway, we report the isolation and characterization of 3 new genes that encode homologues of GTP cyclohydrolase I (GTP-CH), dihydrofolate synthase /folylpolyglutamate synthase (DHFS/FPGS) and serine hydroxymethy ltransfera se (SHMT). The genes encoding GTP-CH and SHMT are unambiguously assigned to chromosome 12, while that for DHFS/FPGS is tentatively assigned to chromos ome 13. All 3 genes are expressed in blood-stage parasites, yielding transc ripts of which only ca 60-70 degrees (o), is accounted for by coding sequen ce. All 3 of the proteins predicted to be encoded hv these genes display se quence differences compared to the human host homologues that ma) be of fun ctional significance. These data bring the complement of cloned genes that encode activities in the pathway to seven, leaving only the gene encoding d ihydroneopterin aldolase (DHNA) to be identified in the route from GTP to f olate synthesis and folate turnover in the thymidylate cycle.