Human Cdc5, a regulator of mitotic entry, can act as a site-specific DNA binding protein

G(2)/M progression requires coordinated expression of many gene products, b ut little is known about the transcriptional regulators involved, We recent ly identified human Cdc5, a positive regulator of G(2)/M in mammalian cells , We also demonstrated the presence of a latent activation domain in its ca rboxyl terminus, suggesting that human Cdc5 regulates G(2)/M through transc riptional activation. Despite the presence of a DNA binding domain, studies by others have failed to identify a preferential binding site for Cdc5 fam ily members. In addition, Cdc5 recently has been associated with the splice some in several organisms, suggesting that it may not act through DNA bindi ng, We now report the identification of a 12 bp sequence to which human Cdc 5 binds specifically and with high affinity through its amino terminus. We show that this DNA-protein interaction is capable of activating transcripti on. We also used a selection system in yeast to identify human genomic frag ments that interact with human Cdc5, Several of these contained sequences s imilar to the binding site. We demonstrate that these bind human Cdc5 with similar specificity and affinity. These experiments provide the first evide nce that Cdc5 family members can act as site-specific DNA binding proteins, and that human Cdc5 may interact with specific, low abundance sequences in the human genome, This raises the possibility that Cdc5 proteins may parti cipate in more than one process necessary for regulated cell division.