Episomal expression of luciferase with constructs utilizing bidirectional promoters. A: Diagram of bidirectional constructs: introns of var genes PFB1055c and PFC0005w are shown in the top diagram while the 5' regions of calmodulin gene (Pcam) and the stress-inducible gene (Psti) are shown on the bottom. firefly refers to the firefly luciferase gene. 3'denotes the 3'untranslated region of the Pfhrp2 gene. The total construct lengths are 7162 bp, 7254 bp and 7315 bp for plasmids containing the PFB1055c, PFC0005w and Pcam/Psti promoters, respectively. B: P. falciparum NF54 stably transformed with bidirectional constructs containing var intron PFB1055c (gray), var intron PFC0005w (black) and Psti/Pcam (white) promoters were synchronized by magnetic cell separation and luciferase activity was measured at different time points throughout the asexual cycle. Samples were measured in triplicates and error bars indicate the standard deviation. C: mRNA levels were determined by synthesis of cDNA followed by quantitative realtime PCR analysis. RNA levels of dhfr (dark gray) and luciferase (hatched) were compared to P. falciparum genes sbp1 (black) and msp1 (white). RNA was extracted ~16 h (ring) and 40 h (schizont) post-invasion.