The 5'-upstream sequence of the phospholipase C-gamma 1 (PLC-gamma 1) gene contains several transcriptional regulatory regions. We have studied one of the regions (-551 to -480, named GPE1) which exhibits a strong positive regulatory activity. GPE1 stimulated the transcription when fused to heterologous TATA element in an orientation-dependent manner. The region between -536 and -470 was identified as the protein binding site in GPE1 by the DNase I footprinting method. Electrophoretic mobility shift assays with several competitors revealed three protein binding sites in this region, designated as GES1, GES2, and GES3. The binding sites were -535 GGAGGGGGCG -524, -512 TGTCACTCA -504, and -491 CAATCCA -485, respectively. Mutational analyses suggested that GPE1 binding proteins cooperate with each other to activate the transcription of the PLC-gamma 1 gene. Additionally, immunoblot analyses revealed that the level of PLC-gamma 1 expression was considerably higher in 9 of 11 colorectal carcinomas than in adjacent normal colorectal tissues. In 7 of 9 cases of colorectal carcinomas which express higher level of PLC-gamma 1, the DNA binding activities to GES1, GES2, and GES3 sites also increased when compared with normal tissues. These results suggest that the GPE1 binding proteins might be attributed to the elevated expression of PLC-gamma 1 in colorectal carcinomas and may play important roles in proliferation of colorectal carcinoma cells.