A novel element and a TEF-2-like element activate the major histocompatibility complex class II transactivator in B-lymphocytes

Major histocompatibility complex (MHC) class II molecules play a central ro le in immune responses, and transcription of this family of genes requires the MHC class II transactivator (CIITA). CIITA has four promoters, which ar e transcribed in a tissue-specific manner. CIITA promoter III is constituti vely active in mature B-lymphocytes. This report now describes the minimal 319-base pair promoter region necessary for maximal transcriptional activit y in B-lymphocytes. Ultratiolet light and dimethylsulfate in vivo genomic f ootprinting analyses reveal five occupied DNA sequence elements present in intact B-lymphocytes, Functional analysis of these elements using promoter deletions and site-specific mutations demonstrates that at least two of the sites occupied in vivo are critical for transcriptional activity. In vitro protein/DNA analysis suggests that one of the sites is a TEF-2-like elemen t and the other is occupied by a novel transcription activator. In addition , nuclear factor-1 associates with the promoter both in vivo and in vitro. In myeloma cell lines, loss of CIITA transcription correlates with a comple tely unoccupied CIITA promoter III. These findings suggest that CIITA trans cription in B-lymphocytes is activated through at least two strong promoter elements, while loss of expression in myeloma cells is mediated through ch anges in promoter assembly.