Asymmetric redundancy in CD4 silencer function.

We and others have defined a transcriptional silencer critical for the proper expression of the CD4 gene at all stages of T cell development. In this report, we use biochemical techniques to identify three different factor-binding sites within the CD4 silencer, denoted sites I, II, and III. Using transgenic analyses, we determine that although all three factor-binding sites are important for silencer activity, there is significant redundancy in that the presence of either site II alone, or the combination of sites I and III permits silencer function. Thus, our data indicate that the mechanism of function of the CD4 silencer is extremely complex. Further biochemical analyses indicate that the factor binding to site II has the same sequence specificity as a factor binding to an E box site in the CD4 enhancer; thus, a member of the bHLH factor family may be important in mediating silencer function.