An environmental agent that triggers transcription from an operon is a/an:

Options

A. Depressor
B. Controlling elements
C. Regulator
D. Inducer

Answer

To solve the question "An environmental agent that triggers transcription from an operon is a/an:", we need to understand the concepts of operons and the factors that influence transcription.

### Step-by-Step Solution:

1. **Understanding Transcription**:
- Transcription is the process of synthesizing RNA from a DNA template. In prokaryotes, this often occurs in structures called operons, which are clusters of genes controlled by a single promoter.

2. **Identifying Environmental Agents**:
- Environmental agents can influence whether genes are expressed or not. These agents can include molecules that bind to specific sites on the operon and affect transcription.

3. **Examining Options**:
- The question presents several options: de-repressor, depressor, regulator, and inducer. We need to evaluate each option to determine which one correctly describes an environmental agent that triggers transcription.

4. **Analyzing Each Option**:
- **De-repressor**: This term refers to the removal of a repressor protein that inhibits transcription. While it plays a role in allowing transcription to occur, it is not an environmental agent itself.
- **Depressor**: This is a protein that binds to the operator region of an operon to inhibit transcription. It does not trigger transcription but rather prevents it.
- **Regulator**: This term typically refers to molecules that can enhance or suppress gene expression but does not specifically denote an environmental agent that triggers transcription.
- **Inducer**: An inducer is a specific type of environmental agent that binds to a repressor or other regulatory proteins, leading to the activation of transcription from the operon.

5. **Conclusion**:
- The correct answer is **inducer**. Inducers are environmental agents that trigger transcription from an operon by interacting with the regulatory proteins involved in the transcription process.

### Final Answer:
An environmental agent that triggers transcription from an operon is a/an **inducer**.