Yin Yang 1 is a lipopolysaccharide-inducible activator of the murine 3′ Igh enhancer, hs3

The 3' Igh enhancers, DNase I hypersensitive site (hs) 3B and/or hs4, are required for germline. transcription, and hence, class' switch recombination for multiple isotypes. A number of hs3-binding transcription factors have, been identified by EMSA, including octamer and NF-kappaB family members, and Pax5. We have found that the binding of the transcription factor, Yin Yang 1 (YY1), to hs3 and to the muE1 site of the intronic enhancer, Emu, is induced in primary splenic B cells after similar to48 h in response LPS and other activators of class switch recombination. Transient transfection experiments in B cell lines indicate that YY1 is an activator of hs3. Interestingly, levels of YY1 expression are unchanged in resting and LPS-stimulated B cells. Mixing experiments followed by EMSA showed that a protein present in resting B cells prevented binding of YY1 to DNA. We found that recombinant retinoblastoma protein (Rb) inhibited binding of YY1 to hs3 in a dose-dependent manner, and we have identified complexes of endogenous YY1 with the Rb in resting B cells, but not in LPS-stimulated B cells. A difference in Rb phosphorylation state was also confirmed between resting (GO) B cells and LPS-stimulated B cells. These observations suggest that the interaction of YY1 with hypophosphorylated Rb in resting B cells prevents interaction of YY1 with DNA. After stimulation with class-switching activators, such as LPS, Rb becomes hyperphosphorylated and YY1 is released and can then bind to the hs3 enhancer and Emu.