Fig. 4. Dose–response curve for the activation of the PDF receptor of D. melanogaster by PDF, PDF1-7 and PDF8-18. The ability of peptides to increase intracellular cAMP levels was determined using HEK293 cells co-transfected with the PDF receptor gene (CG13758) and a firefly luciferase reporter gene construct, as described in Materials and methods. Results are expressed as the mean percentage increase in luciferase activity above basal levels from three replicate wells from three independent transfections (s.e.m.=5–20%). The effects of PDF1-7 and PDF8-18 on the activity of intact PDF were tested by varying the dose of PDF whilst keeping the concentration of each PDF fragment constant at 10^–5 mol l^–1. PDF, solid circle; PDF1-7, open circle; PDF8-18, closed triangle; PDF+PDF1-7, open triangles; PDF+PDF8-18, open square.