EMSA shows binding of basic helix-loop-helix-zipper transcription factors to CD20 promoter site 1. (A) A total of 0.5 ng of labeled probe CD20#1 (lanes 1 to 5) or a μE3 probe (6 to 9) were incubated with BJA-B nuclear extracts and the indicated unlabeled competitor probe (200-fold excess). The resultant complexes were separated on a 5% native polyacrylamide gel. (B) The same probes as in panel A were either incubated with in vitro translated TFE3 (lanes 1 to 8) or BJA-B nuclear extracts (lane 9) in the absence (−) or presence (+) of 200-fold excess of unlabeled competitor probe. In vitro translated TFE3 was either incubated with preimmune serum (lanes 3 and 7) or anti-TFE3 antiserum (lanes 4 and 8) before addition of the indicated probe. EMSA was performed as in panel A. (C) USF1 is a major component of complex 1. EMSA in which nuclear extracts from the indicated source have been preincubated with an antiserum to USF1 or USF2 or with preimmune control. Competition with a 200-fold excess of cold probe.