Cis-element dependence and occupancy of the human invariant chain promoterin CIITA-dependent and -independent transcription

The major histocompatibility complex (MHC)-associated invariant chain (Ii) associates with the class II alpha/beta heterodimer during its biosynthesis , inhibiting association of endogenous peptides with the peptide-binding cl eft. It is therefore not surprising that there are significant similarities in regulatory mechanisms controlling the expression of the structural clas s II MHC and Ii genes. One important similarity is that both classes of gen es can be expressed via CIITA-dependent or -independent mechanisms. In this report, we have dissected CIITA-dependent and -independent transcription o f the Ii gene using an isogenic B-LCL cell pair (Jijoye and clone-13) which do or do not express the class II MHC transactivator (CIITA), respectively . Experiments using mutant or deletion constructs of the Ii gene promoter i ndicate that while both the X-box and Ii-kappa B1 elements are critical for CIITA-dependent transcription in B lymphocytes, the Ii-kappa B1 element is of greater importance for CIITA-independent Ii gene transcription, with th e X-box playing a secondary role. Despite these clear differences in cis-el ement dependence of CIITA-dependent and -independent Ii transcription, ther e are only subtle differences in the occupancy of these elements in vivo as assessed by genomic footprinting. These differences are restricted to occu pancy of the X-box and Y-box, with which the RF-X and NF-Y complexes intera ct in Ii-positive cells. This difference in the occupancy of the X-box and Y-box in this cell pair indicates that while protein/protein interactions b etween CIITA and promoter-bound factors stabilize promoter occupancy, these interactions are not absolutely required for occupancy and transcription o f the invariant chain gene. (C) 1999 Elsevier Science Ltd. All rights reser ved.