CBF is a heteromeric mammalian transcription factor that binds to CCAAT sequences in a number of promoters such as the two type I collagen promoters, the albumin promoter, the major histocompatibility complex class II promoter, and others. It is composed of two components, A and B, that are both needed for DNA binding. We have isolated a rat cDNA containing the complete 341-amino acid coding sequence of the B component of CBF. Expression of this cDNA in vitro generates a polypeptide that shows the same dependency on the A component as the native B component in the formation of a complex with a CCAAT-containing DNA. The C-terminal portion of the B component from residue 260 to residue 312 shows a 75% sequence identity with a portion of the Hap2 protein, a component of a heteromeric CCAAT-binding protein in yeast. In contrast, the rest of the protein shows little sequence homology with Hap2, although both proteins contain glutamine-rich domains. In the B component of CBF this domain spans the amino-terminal 60% of the protein, whereas in Hap2 this domain is much smaller. Hence, only a few changes in one domain of this protein were tolerated during evolution between yeast and mammals, whereas the rest of the protein diverged much more extensively.

Maity, S. N., Vuorio, T., & de Crombrugghe, B. (1990). The B subunit of a rat heteromeric CCAAT-binding transcription factor shows a striking sequence identity with the yeast Hap2 transcription factor. Proceedings of the National Academy of Sciences, 87(14), 5378–5382.