The GC box as a silencer

A DNA control sequence GTGGGCGGGGCAAT, or the "GC" box, has been described in the promoter regions upstream of a number of eukaryotic genes transcribed by polymerase II (for review, see Dynan, W.S. and Tjian, R., Nature 316:774, 1985). The "GC" box can occur in single or multiple copies and is the binding site for a protein factor, Sp1, which activates initiation of transcription. We have observed in the rainbow trout protamine gene 3' to the TATA box, three "GC" boxes spaced at 80 bp intervals. The first is 5' to the cap site and possesses the ability to "silence" transcription from the protamine promoter in constructs linking this promoter to the bacterial chloramphenicol acetyl transferase (CAT) coding sequence following transfection to COS-1 cells. A model is proposed to account for the silencing of the protamine gene in all tissues except developing sperm cells.