Fig. 5

An S–N (N67-N67C) amino acid substitution in the dihydrofolate reductase-thymidylate synthase (DHFR-TS) confers resistance to pyrimethamine. A Sequence alignment of DHFR-TS proteins from Plasmodium y. nigeriensis N67 and N67C parasites. The yellow highlighted sequence is the DHFR signature domain, and the light blue highlighted sequence is the TS active site as determined by ScanProsite (https://prosite.expasy.org/cgi-bin/prosite/scanprosite/ScanView.cgi?scanfile=79818089679.scan.gz). The S to N substitution is marked in red. B Plots of parasitemia after treatment of N67-infected mice with two dosages of pyrimethamine (PYR). Mice were injected with 1 × 10.⁶ iRBCs. From day 3 to day 6 (arrowheads), mice were also IP-injected with PYR (4.2 mg/kg or 12.5 mg/kg) in 10% DMSO and 90% PBS). Parasitemias were counted under a microscope after Giemsa staining. C Plots of parasitemia after treatment of N67C-infected mice with two dosages of PYR as done in (B). (D and E) Plots of parasitemia in mice infected with N67 (D) and N67C (E) after treatment with a higher dosage of PYR (75 mg/kg). Note: Non-treated mice died on day 7 post-infection with N67C, whereas some treated mice survived to day 15; no parasitemia was obtained after day 7 post-infection for these two groups. Mann–Whitney U test (n = 5); *, P < 0.05; **, P < 0.01