Promoter analysis of the G. Intestinalis core histone genes. Luciferase activities were determined from Giardia transfections with a dual-luciferase reporter system. Each Giardia sample was co-transfected with an experimental plasmid and the control plasmid, and assayed sequentially for firefly and Renilla luciferase activities. The firefly luciferase activity was divided by the Renilla luciferase activity to obtain the F/R-LUC ratio. Percentages of relative luciferase activity were calculated by comparing the F/R-LUC ratio obtained from Giardia transfected with each construct relative to the ratio obtained upon transfection of the control plasmid in each experiment. A, Identification of the histone H4 promoter. Experimental plasmids contained incremental deletions of the upstream region of the H4 gene to drive the expression of the firefly luciferase (F-LUC) gene. The composition of the experimental constructs are represented by: white bar, 5' noncoding region of the H4 gene; grey bar, him sequence; black bar, AT-rich sequence; open box, firefly luciferase coding region. The numbers proximal to the white bars indicate the length of 5' noncoding region of the H4 gene remaining within each plasmid. B, Mutational analysis of the histone H4 promoter. Experimental plasmids contained mutations within the 50 bp promoter region of the histone H4 gene to drive the expression of the firefly luciferase (F-LUC) gene. In the wild-type H4 promoter sequence presented on the top line, the him is indicated by the grey box; the AT-rich sequence is indicated by the open box; the g-CAB elements are underlined; and the transcriptional start site is indicated by the bent arrow. C, Comparison of the four core histone promoters. The minimal 5' noncoding sequence of each core histone gene that contain the him sequence was used to drive the expression of the firefly luciferase gene in the experimental plasmid constructs. The him sequences are indicated by grey boxes; the AT-rich sequences are indicated by open boxes; and the g-CAB elements are underlined.