The Transcription Complex

Basic Cis-acting Sequences

Enhancers and Silencers


Trans-acting Factors

Signaling Control of Transcription

Hormonal Control of Gene Expression

Signal Transduction Pathways

Study Questions

Control of Gene Expression in Eukaryotes WWW Links

Genetic Topics

Trans-acting Factors

Trans-acting factors functionally have two domains. One domain is required for the factor to bind to DNA, and the second domain is required for the activation of transcription. This was discovered by studying deletion mutants of the factors. Mutants factors were found that could bind DNA but could not activate transcription. Other experiments in which a hybrid protein consisting of the non-DNA binding segment of one trans-acting factor fused to the DNA-binding region of a second trans-acting activated transcription defined the second function of trans-acting factors. These two experimental results confirmed the existence of two trans-acting factors functions.

Several different classes of trans-acting factors have been described by sequencing the genes and studying their functions. The general feature of all of these proteins is they activate transcription by binding to DNA in the promoter. The table below provides the important details of the these factors.

Class Monomer or Dimer Feature Examples
Helix-turn-helix Monomer Three helix domains; helix 3 binds DNA and helixes 1 and 2 bind to other proteins Drosophila homeotic proteins
Zinc Finger Monomer Bind zinc in a histidine-cysteine rich region of the protein Steroid receptor proteins
Leucine Zipper Homo- or heterodimers Dimers formed through the interaction of leucine-rich regions of the proteins Proto-oncogenes c-Fos and c-Jun; op2 gene of corn
Helix-loop-helix Dimer Two helixes linked by a loop Proto-oncogene c-myc