Transcriptional regulation by the Sp family proteins.

Sp1 is one of the very first cellular transcription factors to be identified and cloned in virtue of its binding to a G-rich motif in the SV40 early promoter. Sp1 protein binds to the G-rich sequences present in a variety of cellular and viral promoters and stimulates their transcriptional activity. Recently, a number of other GC and/or GT box-binding factors homologous to Sp1 have been isolated, namely Sp2, Sp3 and Sp4, and the two more distantly related factors, BTEB and BTEB2. The discovery of this family highlights a previously unknown level of complexity of transcriptional regulation of promoters containing GC and/or GT box motifs. This review focuses primarily on strategies aimed to elucidate the transcription properties of the Sp1-like factors and discusses the experimental problems inherent in the attempt to define their respective functions.