Promoter control of merozoite specific gene expression. (A) Genes upregulated specifically in the merozoite stage (color of individual gene expression profiles based on the Mc52 expression). Three candidate genes for use in cloning merozoite specific promoters are highlighted (blue). (B) Transgenic GRA1, MSF and SRS22B strains were assessed for expression of DHFR-Ty at the tachyzoite (α-Ty + Alexa Fluor 488, yellow and α-SAG1-Alexa Fluor 594, red) and bradyzoite (α-Ty + Alexa Fluor 488, yellow and α-BAG5 + Alexa Fluor 594, red) stages by IFA. Strong nuclear expression of DHFR-Ty was observed for the GRA1 strain and lacked expression for MSF and SRS22B strains at both stages, which tracks with the microarray expression data (Figure 4A and Figure 6A). (C) The MSF strain was assessed for enteric stage expression of DHFR-Ty in the felid intestine (α-Ty + FITC, yellow and α-RH-parasite + TRITC, red). Non-transgenic TgNmBr1 parasite was used as a negative control. Expression of DHFR-Ty was observed for the MSF strain in the enteric stage, confirming stage specific control of expression by the promoter.