Description:
<jats:p>Site‐directed mutagenesis of <jats:italic>cis</jats:italic>‐regulatory elements in the 5′ flanking region of the rat pyruvate kinase M gene revealed that two out of the three GC boxes (–1337–124 and–48/–39) are involved in the stimulation of a core promoter (–35/+46). These two regions were also protected in DNasel foot‐printing assays. Sp1 and Sp3 were identified as binding proteins to all three GC boxes by supershift experiments.</jats:p><jats:p>Cotransfections in <jats:italic>Drosophila</jats:italic> SL2 cells revealed a strong stimulatory function of Sp1 and a synergistic effect of Sp3 to Sp1 in the activation of the pyruvate kinase M promoter. No inhibitory effect of Sp3 was detected.</jats:p><jats:p>These data indicate that binding of Sp1 at two GC boxes is required for full promoter activity of the pyruvate kinase M gene and thus contributes to the observed cell‐cycle‐dependent expression of this enzyme in proliferating rat thymocytes.</jats:p>