The human proto-oncogene product, c-Jun, is a member of the AP-1 family of transcription factors, which mediate the regulation of gene expression in response to extracellular signaling. Comparison of c-Jun and v-Jun by in vitro transcription assays revealed that v-Jun has significantly greater transcriptional activity than c-Jun. Analysis of Jun mutants expressed in bacteria indicates that this difference in transcriptional activity is due to the presence of a regulatory domain located at the N-terminal region of c-Jun. Other Jun mutants identify an activation domain rich in acidic and proline residues toward the C-terminal end of the molecule, in a region near the DNA binding domain. These findings suggest that during retroviral transduction, a constitutively active Jun protein has been generated by deleting a negatively acting domain. This putative repressor domain may also play a role in the signal-dependent induction of c-Jun activity.