NF-X, a transcription factor implicated in MHC class II gene regulation.

The X box has been shown in several assay systems to be a critical element of MHC class II gene promoters. Several X box-binding activities have been discovered in nuclear extracts from a variety of cell lines. The critical question is: which of these are responsible for mediating X box function? This report provides a further characterization of NF-X, a highly specific X box-binding activity we described previously. The cell-type distribution, structural features, and binding site characteristics of NF-X are analyzed in detail, to facilitate comparison with other reported activities. Most importantly, the functional relevance of NF-X is assessed by scanning mutagenesis, and the results indicate that this complex is indeed involved in regulating MHC class II gene expression. With these data in mind, the relationship between NF-X and RF-X, an X box-binding activity reported to be absent in patients with severe combined immunodeficiency, is discussed.