It has been previously proposed that the transcription complexes TFIID and SAGA comprise a histone octamer-like substructure formed from a heterotetramer of H4-like human hTAF_II80 (or itsDrosophila melanogaster dTAF_II60 and yeast [Saccharomyces cerevisiae] yTAF_II60 homologues) and H3-like hTAF_II31 (dTAF_II40 and yTAF_II17) along with two homodimers of H2B-like hTAF_II20 (dTAF_II30α and yTAF_II61/68). However, it has not been formally shown that hTAF_II20 heterodimerizes via its histone fold. By two-hybrid analysis with yeast and biochemical characterization of complexes formed by coexpression in Escherichia coli, we showed that hTAF_II20 does not homodimerize but heterodimerizes with hTAF_II135. Heterodimerization requires the α2 and α3 helices of the hTAF_II20 histone fold and is abolished by mutations in the hydrophobic face of the hTAF_II20 α2 helix. Interaction with hTAF_II20 requires a domain of hTAF_II135 which shows sequence homology to H2A. This domain also shows homology to the yeast SAGA component ADA1, and we show that yADA1 heterodimerizes with the histone fold region of yTAF_II61/68, the yeast hTAF_II20 homologue. These results are indicative of a histone fold type of interaction between hTAF_II20-hTAF_II135 and yTAF_II68-yADA1, which therefore constitute novel histone-like pairs in the TFIID and SAGA complexes.

PC4 is a nuclear DNA‐binding protein that stimulates activator‐dependent class II gene transcription in vitro . Recent biochemical and X‐ray analyses have revealed a unique structure within the C‐terminal domain of PC4 that binds tightly to unpaired double‐stranded (ds)DNA. The cellular function of this evolutionarily conserved dimeric DNA‐binding fold is unknown. Here we demonstrate that PC4 represses transcription through this motif. Interaction with melted promoters is not required for activator‐dependent transcription in vitro . The inhibitory activity is attenuated on bona fide promoters by (i) transcription factor TFIIH and (ii) phosphorylation of PC4. PC4 remains a potent inhibitor of transcription in regions containing unpaired ds DNA, in single‐stranded DNA that can fold into two antiparallel strands, and on DNA ends. Our observations are consistent with a novel inhibitory function of PC4.