DNA-BINDING SITE OF THE YEAST HETEROMERIC INO2P INO4P BASIC HELIX-LOOP-HELIX TRANSCRIPTION FACTOR - STRUCTURAL REQUIREMENTS AS DEFINED BY SATURATION MUTAGENESIS/

The inositol/choline-responsive element (ICRE) is an 11 bp cis-activat ing sequence motif with central importance for the regulated expressio n of phospholipid biosynthetic genes in the yeast Saccharomyces cerevi siae. The ICRE containing the CANNTG core binding sequence (E-box) of basic helix-loop-helix (bHLH) regulatory proteins is recognized by the heteromeric bHLH transcription factor Ino2p/Ino4p, In this study, we define the Ino2p/Ino4p consensus binding sequence (5'-WYTTCAYR-TGS-3') based on the characterization of all possible single nucleotide subst itutions. Interestingly, this analysis also identified a single functi onal deviation (CACATTC) from the CANNTG core recognition element of b HLH proteins, The DNA binding specificities of different yeast bHLH pr oteins may now be explained by distinct nucleotide preferences especia lly at two positions immediately preceding the CANNTG core motif.