DNA-binding proteins for transcription enhancing region of HLA class I gene

Class I regulatory complex (CRC) located in the 5'-upstream region of MHC class I gene contains transcriptional enhancing sequences, called Enh A. This Enh A region contains tandem-arranged kappa B-like sites, one of which has a well-conserved perfect palindromic sequence. The second kappa B-like site, juxtaposed to the perfect palindrome, contains an imperfect palindromic sequence. In B-cell nuclear extracts, we have identified at least four sequence-specific protein complexes; three shared the repeated kappa B enhancer as their binding motifs. The perfect palindromic sequence facilities the binding of a complex termed BI, while the imperfect palindrome provides the binding sites for two other complexes, BII and BIII. The BII and BIII complexes exhibited binding crossreactivity with other kappa B-related motifs and recognized both the perfect and imperfect palindromic sequences, whereas the BI complex was specific for the perfect palindromic sequence which is unique to the class I promoters. A DNA segment outside the repeated kappa B enhancers probably binds the fourth complex, BIV. These complexes, except for the perfect palindrome-binding complex, differ from those described for the murine class I promoter. The binding characteristics of these factors suggest that the mechanism controlling the class I transcription may be quite complex.