Regulation of SV40 early gene expression.

The early promoter of the simian virus 40 (SV40) has been used as a model eukaryotic promoter for the study of DNA sequence elements and cellular factors that are involved in transcriptional control and initiation. Site-directed mutagenesis and cell-free transcription systems have enabled the dissection of the functional domains within the 21 bp upstream sequence element and the 72 bp enhancer, and a number of protein factors that bind to various "motifs" within these domains have been identified. This article summarizes recent observations that have led to the conclusion that the SV40 promoter, and particularly, the enhancer region, is composed of multiple sequence elements. Some of these elements are present in cellular genes, and may exhibit tissue-specificity in their action. Furthermore, the proteins that are being identified (e.g., Sp1) may have binding sites within these elements that are sufficiently specific to ensure that only certain sets of genes will be selectively expressed.