GENE-REGULATORY PROPERTIES OF MYC HELIX-LOOP-HELIX LEUCINE-ZIPPER MUTANTS - MAX-DEPENDENT DNA-BINDING AND TRANSCRIPTIONAL ACTIVATION IN YEAST CORRELATES WITH TRANSFORMING CAPACITY

Max is a basic helix-loop-helix/leucine zipper (bHLH/LZ) protein that forms sequence-specific DNA-binding complexes with the c-Myc oncoprote in (Myc). Using Saccharomyces cerevisiae, we have shown that the Max b HLH/LZ domain enables Myc to activate transcription through CACGTG and CACATG sequences in vivo, and that the number and context of such sit es determines the level of activation. In addition, we have used yeast to investigate the role of the Myc helix-loop-helix (HLH) and leucine zipper (LZ) motifs in mediating Max-dependent DNA-binding and transcr iptional activation in vivo using HLH/LZ mutants generated by site-dir ected mutagenesis. The results show that, while both motifs are essent ial for Myc to activate transcription, helix 2 of the HLH together wit h the contiguous LZ suffice to mediate complex formation with Max, whi lst helix 1 is essential for sequence-specific DNA binding of Myc-Max complexes. Furthermore, the ability of Myc HLH/LZ mutants to hind DNA and activate transcription in collaboration with Max correlates closel y with their neoplastic transforming activity in higher eukaryotic cel ls.