REPRESSION OF MAJOR HISTOCOMPATIBILITY COMPLEX I-A-BETA GENE-EXPRESSION BY DBPA AND DBPB (MYB-1) PROTEINS

The induction of major histocompatibility complex class II gene expres sion is mediated by three DNA elements in the promoters of these genes (W, X, and Y boxes), The U box contains an inverted CCAAT box sequenc e, and the binding activity to the CAAT box is mediated by factor NF-Y , which is composed of subunits NF-YA and NF-YB, We have found that tr ansfection of either dbpA or dbpB (mYB-1) or both inhibits I-A beta ge ne expression, Although the genes for some members of the Y-box family of binding proteins have been isolated by screening an expression lib rary using the Y-box sequence, under our conditions no binding of dbpA or dbpB to the Y box of the I-A beta or I-E alpha promoter was detect ed, This suggested that repression of I-A beta gene expression by dbpA and dbpB was not due to competition for binding to the Y-box sequence , The results suggest two other mechanisms by which dbpA and dbpB can inhibit transcription from the I-A beta promoter. When dbpA was added, the binding of NF-YA to DNA increased, which could be explained by in teraction between these two proteins whose purpose is to increase the binding affinity of NF-YA for DNA. However, this complex was unable to stimulate transcription from the I-A beta promoter, Thus, dbpA compet ed for the interaction between NF-YA and NF-YB by binding to NF-YA, Wh en dbpB factor was added together with NF-YA and NB-YB, the binding of the NF-YA-NF-YB complex was reduced. This suggested that dbpB may com pete with NF-YB for interaction with NF-YA, These results provide an e xample of how dbpA and dbpB may regulate transcription of promoters th at utilize NF-Y as a transcription factor.