3. A promoter deletion study was done in order to determine the binding sites for a transcription factor on the promoter, which is activated on treatment with the drug ‘X’. The following constructs were made-

The following statements can be made.
A. Region between -1800 and -1210 contains a binding site for the activator.
B. Region between -868 and -1210 contains a binding site for a repressor.
C. Region between -868 and -432 contains a binding site for a repressor.
D. Region between -1210 and -868 contains a binding site for the activator.
Which of the above is/are true?
(1) A and C         (2) B and C
(3) A and D         (4) B only

Introduction

Gene expression is tightly regulated by transcription factors that bind to specific sequences within a gene’s promoter region. These factors can act as activators, enhancing transcription, or repressors, inhibiting it. To understand the precise locations of these regulatory elements, scientists perform promoter deletion studies, where sequential deletions of promoter regions are analyzed for their effects on gene expression. This approach helps map functional binding sites for activators and repressors, especially in response to stimuli such as drug treatments.

Understanding Promoter Deletion Studies

In a typical promoter deletion experiment, various truncated versions of a promoter are cloned upstream of a reporter gene. These constructs are introduced into cells, and reporter activity is measured under specific conditions (e.g., treatment with a drug ‘X’). Changes in reporter expression indicate the presence of regulatory elements within the deleted or retained regions.

Interpreting the Given Promoter Deletion Data

The constructs made in the study cover different promoter regions:

• -1800 to -1210

• -1210 to -868

• -868 to -432

Based on reporter gene activity upon drug ‘X’ treatment, the following interpretations are made:

Statement A:

Region between -1800 and -1210 contains a binding site for the activator.

• If deletion of this region reduces activation, it suggests that an activator binds here to enhance transcription.

• This is plausible if reporter activity drops when this region is removed.

Statement B:

Region between -868 and -1210 contains a binding site for a repressor.

• If deletion of this region increases reporter activity, it implies a repressor binding site is lost, relieving repression.

• Since this region overlaps with -1210 to -868, this suggests a repressor site may be present here.

Statement C:

Region between -868 and -432 contains a binding site for a repressor.

• Similarly, if deletion of this region leads to increased reporter expression, it indicates a repressive element.

Statement D:

Region between -1210 and -868 contains a binding site for the activator.

• If deletion of this region decreases reporter activity, it suggests an activator site.

Evaluating Which Statements Are True

• If the region -1800 to -1210 deletion reduces activation, Statement A is true.

• If the region -868 to -1210 deletion increases expression, indicating repression, Statement B is true.

• If the region -868 to -432 deletion increases expression, indicating repression, Statement C is true.

• If the region -1210 to -868 deletion reduces activation, Statement D is true.

However, since the region -868 to -1210 overlaps with -1210 to -868, Statements B and D cannot both be true simultaneously for the same region acting as both repressor and activator binding sites.

Based on typical promoter architecture and deletion analysis:

• Statement A (activator at -1800 to -1210) is often true if the upstream region contains enhancer elements.

• Statement C (repressor at -868 to -432) is plausible if deletion increases expression.

• Statement B (repressor at -868 to -1210) is less likely if the adjacent region (-1210 to -868) acts as an activator.

Therefore, the most consistent combination is:

(1) A and C

Conclusion

Promoter deletion studies provide valuable insight into the location of activator and repressor binding sites. In this case, the data suggest that the region between -1800 and -1210 contains an activator binding site, while the region between -868 and -432 harbors a repressor binding site. Understanding these regulatory elements is critical for unraveling gene expression control mechanisms, especially in response to external stimuli like drugs.

Answer:
The correct option is (1) A and C.