A MINIMAL REGULATORY REGION MAINTAINS CONSTITUTIVE EXPRESSION OF THE MAX GENE

Max is a basic helix-loop-helix/leucine zipper protein that forms hete rodimers with the Myc family of proteins to promote cell growth and wi th the Mad/Mxi1 family of proteins to inhibit cell growth. The role of Max as the obligate binding partner for these two protein families ne cessitates the observed constitutive expression and relatively long ha lf-life of the max mRNA under a variety of growth conditions. In this study, we have used the chicken max gene to map DNA elements maintaini ng max gene expression in vertebrate cells. We have identified a minim al regulatory region (MRR) that resides within 115 bp of the max trans lation initiation site and that possesses an overall structure typical of TATA-less promoters. Within the MRR are two consensus binding site s for Spl, a ubiquitously expressed transcription factor that plays a role in the expression of many constitutive genes. Interestingly,,ve s how that direct binding by Spl to these sites is not required for MRR- mediated transcription. Instead, the integrity of a 20-bp DNA element in the MRR is required for transcriptional activity, as is the interac tion of this DNA element,vith a 90-kDa cellular protein. Our data sugg est that it is the persistence of this 90-kDa protein in vertebrate ce lls which drives max gene expression, insulates the max promoter from the dramatic changes in transcription that accompany cell growth and d evelopment, and ensures that adequate levels of Max will be available to facilitate the function of the Myc, Mad, and Mxi1 families of prote ins.