The X1 box of HLA-G promoter is a target site for RFX and Sp1 factors.

HLA-G gene regulation was investigated with regards to homologies among the pathways regulating both classical MHC class I and MHC class II gene expression. They include four conserved cis-acting regulatory elements located in the proximal promoter region referred to as the W/S/Z box, the X box that is comprised of the X1 and X2 halves, and the Y box with an inverted CCAAT site. The X1 box is the binding site for the ubiquitous RFX complex consisting of three subunits; the X2 box is bound by the X2BP/ATF/CREB family factors. The basic S-X-Y regulatory module interacts with CIITA, which is expressed constitutively in APCs, but may be inducible in others cell types by IFN-gamma. Within HLA-G gene promoter the only conserved motifs are S and X1 boxes. We thus investigated the binding capacity of the HLA-G X box in comparison to that of HLA-DRA and HLA-E. We demonstrate that X2 box mutations in HLA-G promoter affect the binding of ATF/CREB family factors and may privilege the X2 box to access by other shared factors. The X1 box is the target for RFX complex and an additional factor we identified as Sp1. We propose that the X region in the HLA-G gene promoter might participate to the combination of factors which play a role in HLA-G gene activation.