The second kappa B element, kappa B2, of the HLA-A class I regulatory complex is an essential part of the promoter.

The MHC class I regulatory complex (CRC) contains the well characterized palindromic kappa B element, kappa B1, to which a number of nuclear factors are known to bind. This cis element plays an important role in controlling the transcription of MHC class I genes. In addition, the CRC contains a second kappa B element, kappa B2, which is located in tandem with the kappa B1 site, 5 bp upstream. In this study, from reporter gene transfection experiments, we present evidence that this kappa B2 site is as important as kappa B1 in regulating HLA-A locus transcription. Mutations introduced into either kappa B site reduced promoter activity to approximately the level obtained when the entire CRC was deleted. Electrophoretic mobility shift and supershift assays showed that the kappa B2-binding complex (BIII) contained a p65-like subunit, but apparently not the p50 subunit of NF-kappa B. Complex BIII also bound to the kappa B1 site, but the kappa B2 sequence was poorly recognized by the kappa B1-binding complex, BI, probably KBF1, the p50 homodimer. These results demonstrate that both kappa B sites are essential components of the promoter and suggest that they may function together to control MHC class I gene transcription.