Screening of the regulatory elements related to the tissue-specific expression of the PF4 gene. (A) Relative activities of the constructs containing PF4 promoter fragments in the 4 cell lines. HepG2 and HeLa cells are nonmegakaryocytic cell lines. K562 cells are inducible, and HEL cells are constitutive megakaryocytic cell lines. Reporter plasmids containing either a full-length PF4 promoter (P2-PF4) or each of 7 fragments (P2A-P2G) linked to the SV40 minimal promoter were transfected into the 4 cell lines, and the luciferase assay was performed. Luciferase activities are expressed as relative activities ± SD, where the activity of the PGV-P2 was regarded as 100. (B) Structure of the novel regulatory element found only in the P2-E. Characteristic sequences are indicated by the lines above (repeated sequence, thin line) and below (ETS-1–like motif, thick line) the DNA sequence. Dotted lines and boxed sequences indicate E-box motifs and MEIS1 (TGACAG) binding motifs, respectively.