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Induction of MHC Class II Genes in G1B Cells: Identification of a Novel and Functionally Critical Leucine-Rich Motif (62-LYLYLQL-68) in the Regulatory Factor X 5 Transcription Factor1

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UNC Lineberger Comprehensive Cancer Center and Department of Immunology and Microbiology and
Curriculum in Oral Biology, School of Dentistry, University of North Carolina, Chapel Hill, NC 27599
MHC class II deficiency found in bare lymphocyte syndrome patients
results from the absence or dysfunction of MHC class II transcriptional
regulators, such as regulatory factor X (RFX) and class II
transactivator (CIITA). Understanding the roles of these factors has
been greatly facilitated by the study of genetic defects in cell lines
of bare lymphocyte syndrome patients, as well as in cell lines that
have been generated by chemical mutagenesis in vitro. The latter group
includes MHC class II-deficient lines that are no longer responsive to
induction by IFN-
. Here, we show that the defect in G1B, one such
cell line, is attributed to the lack of functional RFX5, the largest
subunit of RFX. The RFX5 gene isolated from G1B cells
contains two separate single-base pair mutations. One alteration does
not exhibit a phenotype, whereas a leucine-to-histidine mutation
eliminates DNA-binding and transactivating functions. This mutation
lies outside of previously defined functional domains of RFX5 but
within an unusual, leucine-rich region (62-LYLYLQL-68). To further
investigate the significance of the leucine-rich region, we targeted
all neighboring leucine residues for mutagenesis. These mutants were
also unable to transactivate a MHC class II reporter gene, confirming
that these leucine residues play an essential role in RFX activity and
characterize a novel leucine-rich motif.
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