Introduction The Transcription Complex Basic Cis-acting Sequences Enhancers and Silencers Methylation 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