ACTIVATION OF CLASS-II MHC GENES REQUIRES BOTH THE X-BOX REGION AND THE CLASS-II TRANSACTIVATOR (CIITA)

CIITA, a gene that can complement a transcriptional mutation of the ma jor histocompatibility complex (MHC) class II genes, was tested for it s ability to function as a coactivator. CIITA cDNA clones isolated sho wed alternative RNA splicing, but only one splice site combination was able to restore class II MHC gene expression. DNA-mediated transfecti on experiments showed that CIITA directs its activity through the X bo x element; the presence of CIITA leads to the formation of a higher or der complex at the X box region; and CIITA contains a potent activatio n domain. These findings support the hypothesis that CIITA directly in teracts with the MHC class II-specific transcription factors and is re quired for expression.