SPECIFICITY IN CHOLESTEROL REGULATION OF GENE-EXPRESSION BY COEVOLUTION OF STEROL REGULATORY DNA ELEMENT AND ITS BINDING-PROTEIN

When demand for cholesterol rises in mammalian cells, the sterol regul atory element (SRE) binding proteins (SREBPs) are released from their membrane anchor through proteolysis. Then, the N-terminal region enter s the nucleus and activates genes of cholesterol uptake and biosynthes is. Basic helix-loop-helix (bHLH) proteins such as SREBPs bind to a pa lindromic DNA sequence called the E-box (5'-CANNTG-3'). However, SREBP s are special because they also bind direct repeat elements called SRE s, Importantly, sterol regulation of all promoters studied thus far is mediated by SREBP binding only to SREs, To study the reason for this we converted the direct repeat SRE from the sterol-regulated low-densi ty lipoprotein receptor promoter into an E-box, In this report we show that SREBPs are still able to bind and activate this promoter however , sterol regulation is lost. The results are consistent with the mutan t promoter being a target for prormiscuous activation by constitutivel y expressed E-box binding bHLH proteins that are not regulated by chol esterol. Kim and coworkers [Kim, J. B., Spotts, G. D., Halvorsen, Y.-D ., Shih, H.-M., Ellenberger, T,, Towle, H, C, & Spiegelman, B, M, (199 5) Mel. Cell. Biol, 15, 2582-2588] demonstrated that the dual DNA bind ing specificity of SREBPs is caused by a specific tyrosine in the cons erved basic region of the DNA binding domain that corresponds to an ar ginine in all other bHLH proteins that recognize only E-boxes, Taken t ogether the data suggest an evolutionary mechanism where a DNA binding protein along with its recognition site have coevolved to ensure maxi mal specificity and sensitivity in a crucial nutritional regulatory re sponse.