THE C-MYC PROTEIN REPRESSES THE LAMBDA-5 AND TDT INITIATORS

The lambda 5 promoter initiates transcription at multiple sites and co nfers expression in all cell types. Two lambda 5 promoter-derived olig onucleotides (Inr(lambda 5:1) and Inr(lambda 5:2)), each with a transc ription start site, could promote transcription in transient transfect ion assays. In contrast, a third oligonucleotide (+90(lambda 5)), with out a transcription initiation site, was inactive. The Inr(lambda 5:1) and Inr(lambda 5:2) oligonucleotides formed a major DNA-protein compl ex B' in gel retardation analyses; no protein-DNA complexes were obser ved with the inactive +90(lambda 5) oligonucleotide. The B' complexes of Inr(lambda 5:1) and Inr(lambda 5:2) each contained c-myc and myn (m urine homologue of Max) proteins. The c-myc and myn proteins were also found to bind the TdT initiator (Inr(TdT)) Using mutated oligonucleot ides, we found that the c-myc/myn proteins bound to the transcription initiation site of both Inr(lambda 5:1) and Inr(TdT), however, these m utated oligonucleotides were inactive in transfection assays. This sug gested that, in this system, transcription depended both on a transcri ption initiation site and appropriate flanking sequences. The signific ance of c-myc binding to the respective initiator was analysed by over expressing c-myc in co-transfection assays. Under these conditions the transcriptional activity of both the lambda 5 and the TdT initiator w as repressed.