Evolution of Sp transcription factors.

The Sp family of transcription factors binds GC-rich DNA sequences. The ubiquitously expressed Sp1 and Sp3 have been well characterized in mammals. Presented here is the characterization of the only Sp protein expressed in the liver or heart tissue of the teleost fish Fundulus heteroclitus. This protein, fSp3, is most similar to and homologous with mammalian Sp3 proteins. The evolution of the Sp transcription family is described, with Sp1 and Sp3 representing the most recent duplication of the Sp family. Sp4 appears to be the most ancestral member. Sp1, Sp3, and Sp4 form a monophyletic group without Sp2. Sp2 is the least similar of the Sp family and is more similar to the non-Sp transcription factors. These results suggest that Sp2 should not be considered a member of the Sp family. Only two domains (zinc fingers and B domain) share similarity outside the Sp family. The zinc fingers are homologous to other GC-binding domains, yet the B domain is homologous to protein-protein interacting domains in the CCAAT-binding/NF-Y transcription factor families. These results suggest that these different domains have different evolutionary histories.