An "attenuator domain" is sandwiched by two distinct transactivation domains in the transcription factor C/EBP.

C/EBP is a rat liver DNA-binding protein which can act as a transcription factor. Its N-terminal portion contains three distinct domains. The first domain (amino acids 1 to 107) appears to be a highly potent transactivator. The second domain (amino acids 107 to 170) does not appear to exhibit either activation or repression activity. This domain is defined as an "attenuator domain" because its presence under four different sequence contexts reproducibly decreases the effect of transactivation of C/EBP. The third domain (amino acids 171 to 245) is a relatively weaker transactivator with a striking proline-rich motif. Deletional analysis of this third domain has shown that a 45-amino-acid region is sufficient for transactivation. This region (amino acids 171 to 215) contains 12 proline, 6 histidine, and mainly hydrophobic or noncharged amino acids. Further mutational analysis of a highly conserved proline-octamer region within this domain indicates that a specific proline content is not crucial for transactivation.