We
have shown that the LEC1 polypeptide is homologous to the HAP3 subunit
of the CBF class of eukaryotic transcriptional activators that includes
NF-Y, CP1, and HAP2/3/4/5 (Johnson and McKnight 1989).
The sequence similarity between LEC1 and other HAP3 subunits is
restricted to the B domain, consistent with the finding that this domain
is conserved evolutionarily (Li et al. 1992).
Furthermore, amino acid residues of yeast and mammalian HAP3 subunits
required for DNA binding and for interactions with other CBF subunits
are conserved in LEC1 (Figure 4;
[55] and [44]).
Experiments demonstrating that yeast and mammalian CBF subunits can be
combined to form DNA-binding complexes indicate that this amino acid
sequence similarity underlies functional conservation ([6] and [43]).CBFs
are heteroligomeric transcription factors, but it is not known whether
the plant CBF is organized as its yeast counterpart into four
nonhomologous subunits, HAP2, 3, 4, and 5, or is similar to the trimeric
mammalian CBF ([24] and [27]).