Eukaryotic transcription factors TF1 and TF2 bind to independent cis-regulatory elements (Cis1 and Cis2, respectively) upstream of the TATA box and positively regulate gene expression. Histone modifier 1 (HM1) binds to TF1 but not TF2. In order to determine how genes are regulated by these three factors, an in vitro transcription and translation assay was set up. A packaged DNA containing Cis1 to Cis2, along with eukaryotic minimal promoter, was purified. The luciferase activity, upon addition of a combination of TF1, TF2 and/or HM1, in the presence of RNA polymerase and translation mix is plotted below.

Which one of the following models best represents the results above?

Correct option is C

Explanation:

• Option 1: TF2 activates luciferase expression independent of TF1 and HM1.

  • This is incorrect because the data shows that luciferase activity is significantly higher when both TF1 and TF2 are present, along with HM1. TF2 alone does not induce high luciferase activity.

• Option 2: HM1 activates binding of either TF1 or TF2 to their cognate Cis elements to activate luciferase expression.

  • This is partially correct, but it doesn’t fully explain the data. HM1 indeed helps in binding TF1 and TF2, but the data shows that the combination of TF1, TF2, and HM1 leads to the highest luciferase activity. It’s not just about activation of binding, but rather facilitating the correct interaction between these factors.

• Option 3: TF1 binds to Cis1, recruits HM1 to modify DNA allowing TF2 to bind Cis2 to enhance luciferase expression.

  • This is the correct explanation. The graph shows that the highest luciferase activity occurs when TF1 is bound to Cis1, and HM1 is recruited, which allows TF2 to bind Cis2, significantly enhancing luciferase expression.

• Option 4: TF2 binds to Cis2, recruits HM1 to modify DNA allowing TF1 to bind Cis1 to activate luciferase expression.

  • This is incorrect because the data shows that TF1 needs to bind Cis1 first for optimal expression, not the other way around.

Information Booster:

1. TF1 binds to Cis1, initiating transcription and recruiting HM1 to modify the DNA.
2. HM1 plays a crucial role in facilitating TF2's binding to Cis2, thus enhancing transcription.
3. The highest luciferase activity is observed when TF1, TF2, and HM1 are all present together, showing the cooperation between these factors.
4. RNA polymerase and translation mix are necessary but do not suffice to increase luciferase expression without the correct interaction between the transcription factors and the histone modifier.
5. The experiment suggests that TF1 and TF2 do not act independently; rather, their interaction through HM1 is key to activating gene expression.
6. Cis1 and Cis2 are cis-regulatory elements, and the presence of both elements is required for optimal transcription factor binding and gene expression.