Although the folate metabolic pathway in malaria parasites is a major chemotherapeutic target, resistance to currently available antifolate drugs is an increasing problem. This pathway, however, includes a number of enzymes that, to date, have not been characterized despite their potential for clinical exploitation. As a step towards evaluation of additional targets in this 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 hydroxymethyltransferase (SHMT). The genes encoding GTP-CH and SHMT are unambiguously assigned to chromosome 12, while that for DHFS/FPGS is tentatively assigned to chromosome 13. All 3 genes are expressed in blood-stage parasites, yielding transcripts of which only ca 60-70% is accounted for by coding sequence. All 3 of the proteins predicted to be encoded by these genes display sequence differences compared to the human host homologues that may be of functional significance. These data bring the complement of cloned genes that encode activities in the pathway to seven, leaving only the gene encoding dihydroneopterin aldolase (DHNA) to be identified in the route from GTP to folate synthesis and folate turnover in the thymidylate cycle.

Thiol-disulphide redox regulation has a key role during the biogenesis of mitochondrial intermembrane space (IMS) proteins. Only the Cys-reduced form of precursor proteins can be imported into mitochondria, which is followed by disulphide bond formation in the mitochondrial IMS. In contrast to the wealth of knowledge on the oxidation process inside mitochondria, little is known about how precursors are maintained in an importcompetent form in the cytosol. Here we provide the first evidence that the cytosolic thioredoxin system is required to maintain the IMS small Tim proteins in reduced forms and facilitate their mitochondrial import during respiratory growth.Keywords: redox regulation; mitochondrial import; thioredoxin; oxidoreductase; folding EMBO reports (2012) 13, 916-922.