In a nutshell, we propose that ZBTB48 is a priming factor that opens up its (few) target genes to then give access for other TFs. In such a model, ZBTB48 acts as an on-off-switch while other TFs fine-tune how much mRNA is produced.

Finally, we wanted to understand how ZBTB48 regulates CIITA (and other genes). In line with transcriptional regulation the H3K4me3 signal at CIITA pIII was entirely dependent on ZBTB48 and upstream of this ZBTB48 was also required for chromatin access as assayed by FAIREseq.

Does this matter at the organismal level? We created a Zbtb48 KO strain and observed that MHC II surface levels in B cells were also strongly reduced, with the most striking effect in pre-B cells. Interestingly, female (but not male) KO mice presented with mild splenomegaly.

To test a functional impact, we induced CIITA/MHCII expression by IFNgamma treatment. In contrast to WT cells, ZBTB48 KO cells had strongly reduced CIITA & MHCII mRNA and protein levels. The CIITA pIII specific transcript was almost entirely shut off.

We biochemically mapped the binding sites and ZBTB48 binds directly to the two critical activating elements, ARE1/2. A corresponding co-crystal structure revealed involvement of ZnF10 & ZnF11 with the former dispensable for ZBTB48's telomeric function --> separation-of-function.