POSITION AND ORIENTATION INDEPENDENT TRANSACTIVATION BY C-MYC

The c-myc oncogene c-Myc is commonly activated in cancer and transacti vates gene expression by binding to CACGTG DNA sequences as a heterodi meric complex with Max. The ornithine decarboxylase (ODC), p53, prothy mosin alpha and ECA39 promoters are transactivated by c-Myc, and are c onsidered direct targets, as activation is mediated by CACGTG sequence s. Interestingly, the c-Myc-responsive CACGTG sequences in the p53, pr othymosin alpha, ECA39 and murine ODC genes are all downstream of the RNA CAP site, suggesting that downstream sequences are preferred c-Myc targets. Using a series of heterologous reporter constructs, we have tested the effects of position and orientation of c-Myc-responsive CAC GTG sequences on c-Myc's ability to activate transcription. A single b inding site conferred c-Myc-responsiveness independent of position and orientation, and over distances of 1.7 kbp. The extent of transactiva tion was not significantly influenced by position of the responsive el ements. By contrast, the extent of transactivation was dependent upon the number of c-Myc binding sites. The results demonstrate that c-Myc activates transcription independent of position and orientation and th at considerable flexibility exists in the interaction of c-Myc transac tivation domains with the general transcription machinery.