Activation of c‐myc promoter by c‐myc protein in serum starved cells

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Abstract
The function of the c‐myc protein, the product of a proto oncogene, is not clearly understood although many reports, including ours, suggest that the c‐myc protein plays several roles in the regulation of transcription and DNA replication. Here we examined the effects of c‐myc protein on transcription from the c‐myc promoter, and by inference its role in auto‐regulation, after introducing into cultured cells a c‐myc expression vector and a CAT reporter gene linked to the promoter and upstream region of the human c‐myc gene. To minimize the effects of the endogenous c‐myc protein on the exogenously added CAT reporter gene, the transfected cells were treated under serum‐free conditions. The results show that CAT expression from the myc promoter increased in a dose‐dependent manner after addition of the c‐myc expression vector, and that it also required the presence of a c‐myc binding sequence previously identified 2 kb upstream from c‐myc's first exon. Moreover, the domains of the c‐myc protein important for transactivation were determined by use of various deletions mutants of c‐myc cDNA. The results showed that the N‐terminal portion in the c‐myc protein was necessary for transactivation beside the C‐terminal portion containing basic region, helix‐loop‐helix, and leucine zipper.