Coordinate regulation of HLA class II genes: a novel DNA binding complex.

We have characterized a nuclear protein complex from B lymphoblastoid cell lines that binds to HLA class II promoters as detected by electrophoretic gel mobility shift assays (EMSA). This complex (C1) binds to three independent sites in the proximal DRA promoter which have not been identified previously as cis-acting elements. C1 is very abundant in Burkitt's lymphoma cell lines, but less abundant in "normal" B lymphoblastoid cell lines. The binding specificity of the C1 complex was analysed using competition experiments and chemical footprinting methods. Complexes with specificity similar to C1 also bind the DPA and DQA promoters. Though mutation of the sequences in the DRA promoter that severely reduced binding of the C1 complex had no effect on the ability of the DRA fragment to drive transcription of the reporter gene in transient expression or in vitro transcription assays, this conservation of binding sites among all class II promoters tested suggests functional relevance in transcription. In addition, complexes similar to C1 were observed in nuclear extracts from all cell lines examined, but minor differences in mobility appeared to correlate with class II expression. Thus, the C1 complex may act as a trans-acting factor in MHC class II expression.