IDENTIFICATION OF A CDNA FOR SSRP1, AN HMG-BOX PROTEIN, BY INTERACTION WITH THE C-MYC ONCOPROTEIN IN A NOVEL BACTERIAL EXPRESSION SCREEN

We describe a system for screening cDNA expression libraries in Escher ichia coli based on protein-protein interactions. The system utilizes fusion proteins containing the DNA binding domain of the lambda phage cl repressor and a heterologous dimerization domain, which is the targ et of the screen. Such chimeric proteins were functional as transcript ional repressors in E.coli; function was dependent on the presence of the heterologous dimerization domain, and function of the chimeras was disrupted by expression of excess dimerization domain. A screen was d esigned to identify factors that could interact with the heterologous dimerization domain and thereby inactivate the chimeric repressor. We used this screen to identify factors that could interact with the basi c helix-loop-helix/leucine zipper domains of c-Myc, and isolated the c DNA for a previously characterized HMG domain protein that interacts s pecifically with c-Myc in this system. This screening method could be used with proteins that have the ability to homo- or heterodimerize.