Hypomethylation of MHC class II Eb gene is associated with expression

Specific complex formation between the type II bare lymphocyte syndrome-associated transactivators CIITA and RFX5

Two of the genes defective in the five complementation groups identified in the class II-negative bare lymphocyte syndrome or corresponding laboratory mutants have been cloned. One gene encodes a protein, RFX5, that is a member of the RFX family of DNA binding proteins. The other, CIITA, encodes a large protein with a defined acidic transcriptional activation domain; this protein does not interact with DNA. Expression plasmids encoding regions of RFX5 fused to the GAL4 DNA binding domain activated transcription from a reporter construct containing GAL4 sites in a cotransfection assay in the Raji human B cell line. However, these plasmids produced transcriptional activity in HeLa cells only in conjunction with interferon gamma stimulation, a condition in which expression of both CIITA and class II major histocompatibility complex surface proteins are induced. Furthermore, these plasmids were not active in RJ2.2.5, an in vitro mutagenized derivative of Raji in which both copies of CIITA are defective. Transcriptional activation by the RFX5 fusion protein could be restored in RJ2.2.5 by cotransfection with a CIITA expression plasmid. Finally, a direct interaction between RFX5 and CIITA was detected with the yeast two-hybrid and far-Western blot assays. Thus, RFX5 can activate transcription only in cooperation with CIITA. RFX5 and CIITA associate to form a complex capable of activating transcription from class II major histocompatibility complex promoters. In this complex, promoter specificity is determined by the DNA binding domain of RFX5 and the general transcription apparatus is recruited by the acidic activation domain of CIITA.

Interaction of H-2Eb with an IAP retrotransposon in the A20/2J B cell lymphoma

In the A20/2J BALB/c B cell lymphoma, Southern analysis revealed an insertion of approximately 6 kilobases of DNA into the first intron of one Ebd-allele. Two observations suggest that the rearrangement did not occur recently in the A20/2J subline. Firstly, normal and altered Ebd-alleles are present in equal numbers, and secondly, the LB 27.4 and LS 102.9 somatic cell hybrids formed at an earlier date both possess the rearrangement. Sequences of two cDNA clones, lambda Eb-7 and lambda Eb-125, selected from an A20/2J cDNA library prepared from poly [A+] RNA indicate that the rearranged Ebd-allele directs the synthesis of atypical Eb transcripts. The clones contain Eb sequence linked to a portion of retroviral-like intracisternal A-particle (IAP) genomic sequence, and they appear to be copies of mRNA produced by splicing between a 5' donor site in the retroviral transcript and the 3' acceptor site of the Eb gene's first intron. The longer lambda Eb-125 insert corresponds to RNA that initiated in the 5'-untranslated region of the Eb gene. The 3'-end of the first Eb exon joins to long terminal repeat sequence, and retroviral sequence extends up to the splice junction with the second Eb exon; 3' of the junction, the lambda Eb-125 sequence corresponds to that of a correctly spliced Eb transcript. It seems feasible that the cDNA clones represent hybrid RNA synthesized by read-through transcription of the Eb coding region and an IAP element inserted into the first intron of the rearranged Ebd-allele.