HLA-B locus-specific downregulation in human melanoma requires enhancer A as well as a sequence element located downstream of the transcription initiation site

Human MHC class I molecules are encoded by three different loci (HLA-A, -B, and -C), which are regulated at the transcriptional level through several conserved cis-acting promoter elements. The presence of locus-specific resi dues throughout the entire promoter region strongly suggests that the vario us HLA class I loci are differentially regulated. To identify regulatory se quences involved in locus-specific HLA class I gene transcription, a series of truncated HLA-A2 and HLA-B7 promoter-reporter constructs were transfect ed into melanoma cell lines expressing high and low levels of endogenous HL A-B, but comparable levels of HLA-A. These experiments showed that differen tial regulation of HLA-B expression in melanoma cell lines is mediated by a previously unidentified co-operative action of enhancer A, located 175 bp upstream of the transcription initiation site (+1), and a specific region o f 20 nucleotides located at +13 to +33 bp downstream of the transcription i nitiation site. Furthermore, we demonstrated binding of transcription facto r Yin Yang 1 to the HLA-A +13/+33 bp region, but not to the equivalent HLA- B region. Based on these results, we present a model suggesting that YY1 di splaces either activating or repressing transcription factors, thereby maki ng the HLA-A gene resistant to differential regulation.