The class II transactivator (CIITA) is a transcriptional master regulator of class II MHC genes, discovered by complementation cloning of an HLA-D-negative mutant cell line. Defective CIITA leads to a type of severe immunodeficiency in humans, the Group A, type II Bare Lymphocyte Syndrome (BLS). CIITA controls the expression of conventional class II MHC genes, as well as DM, Doalpha and Ii genes. It is a potent activator which causes the activation of class II MHC promoters by up to 100 fold. It controls promoters which contain class II MHC promoter elements, W, X and Y, yet it is not a DNA-binding protein. The mechanism by which CIITA activates class II MHC promoters and its novel target genes has been, and continues to be, the focus of this proposal. Experiments performed during the last grant period have shown that CIITA operates by interaction with the DNA- binding transcription factors, NF-Y and RFX, which control class II MHC promoters. Additionally, CIITA opens class II MHC promoters in vivo as determined by genomic footprinting. The purpose of the first half of this proposal is to understand how CIITA control class II MHC genes at the molecular level. The first two Aims continue to investigate how CIITA alters gene expression, and they are: (1) To further examine a HAT-like domain within CIITA which is associated with histone acetylase activity; (2) to determine if CIITA alters other aspects of chromatin opening by studying changes in intact cells. The purpose of the second half of the proposal is to understand if CIITA controls other genes, how this control is executed, and if these target genes have relevant biologic functions. During the last grant period, we show that CIITA may control other genes and biologic processes as revealed by differential gene screen analysis and by comparing primary cells obtained from CIITA gene knockout mice with wildtype controls. To continue this line of inquiry, the last two Aims are (3) to understand the molecular mechanism by which CIITA controls non-class II MHC genes; and (4) determine if these genes are relevant in antigen presenting cells.