The X boxes from promoters of HLA class II B genes at different loci do not complete for nuclear protein-specific binding.

Expression of HLA class II genes is coordinately regulated by cis-acting elements present in their promoter regions immediately upstream from the 5' end of their transcription start sites. Trans-acting factors from the nuclear proteins of the cell are able to positively or negatively regulate transcription of these genes by binding to highly conserved sequences, called boxes. After cloning the promoter regions of all the transcribed class II B genes present in the cell line Priess, we were able to identify certain protein-box complexes and to determine the affinity of these proteins for their respective boxes by comparing promoter boxes of each gene to those of the other genes. Different nuclear proteins seemed to bind to the X boxes of the different class II B genes tested. In the case of the Y box-protein complexes, the various Y boxes competed with similar affinities. The protein(s) which specifically bound to the DRB1-CCAAT box also bound to DPB1-CCAAT box, but completely failed to bind the homologous box from DQB1. Further, CCAAT box-specific protein(s) did not bind to the Y box of the same gene, excluding the possibility that these proteins just recognize the reverse CCAAT box (ATTGG) present within Y.