Two-hybrid analysis of p66 and p50 interactions. β-galactosidase assays were carried out as described in the Methods section using strain Y190 co-transformed with plasmids as indicated. Top panel: various proteins expressed as fusions with the transcription activation domain of GAL4 (pACT) were co-expressed with human p66 expressed as a fusion with the sequence specific DNA-binding domain of GAL4 (pAS). Lower panel: activation domain fusions were co-expressed with human p50 fused with the sequence specific DNA-binding domain of GAL4. pACT constructs are as described: p66 (Methods section); Pcn1 (S. pombe PCNA) ; Cdc27 (S. pombe p66 homologue) ; Fen1 (human repair and replication endonuclease) ; UNG2 (human uracil DNA glycosylase)  and S. cerevisiae Snf1 as a negative control.