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Histones are among the most conserved proteins in evolution, sharing a histone fold motif. A number of additional histonic proteins exist and are involved in the process of transcriptional regulation. We describe here the identification, cloning and characterization of two small members of the H2A-H2B sub-family (YBL1 and YCL1) related to the NF-YB and NF-YC subunits of the CCAAT-binding activator NF-Y and to the TATA-binding protein (TBP) binding repressor NC2. Unlike the latters, YBL1 and YCL1 have no intrinsic CCAAT or TATA-binding capacity. In nucleosome reconstitution assays, they can form complexes with histones in solution and on DNA and they are part of relatively large complexes, as determined by glycerol gradient experiments. Our data support the idea that YBL1 and YCL1 are divergent with respect to NF-YB and NF-YC for specific functions, but have coevolved the capacity to interact with nucleosomal structures.

NF-Y is a sequence-specific evolutionary conserved activator binding to CCAAT boxes with high affinity and specificity. It is a trimer formed by NF-YA and two putative histone-like subunits, NF-YB and NF-YC, showing similarity to histones H2B and H2A, respectively. We investigated the relationships between NF-Y and chromatin using an Artemia franciscana chromatin assembly system with plasmids containing the Major HistoCompatibility complex class II Ea promoter. The NF-Y trimer, but not single subunits, protects the Y box in the presence of reconstituted chromatin, and it can bind the target sequence during and after assembly. Using reconstitution assays with purified chicken histones, we show that NF-Y associates with preformed nucleosomes. Translational analysis of various Ea fragments of identical length in which the CCAAT box is at different positions indicated that the lateral fragment was slightly more prone to NF-Y binding. In competition experiments, NF-Y is able to prevent formation of nucleosomes significantly. These data support the idea that NF-Y is a gene-specific activator with a built-in capacity to interface with chromatin structures.