Lac operon components: Which element listed is not considered part of the lac operon transcription unit itself?

Introduction / Context:
The lac system in E. coli is a model for gene regulation. It consists of a promoter (P), operator (O), and structural genes (lacZYA). The repressor gene lacI is closely linked but transcribed from its own promoter.

Given Data / Assumptions:

• “Operon” refers to the promoter–operator–structural gene cluster transcribed as a single mRNA.
• lacI is regulatory and divergently transcribed.
• Question asks which item is not part of the operon proper.

Concept / Approach:
By definition, the lac operon includes P, O, and lacZYA. The lacI gene encodes the repressor protein but is not within the lacZYA transcription unit; it is therefore not part of the operon.

Step-by-Step Solution:

Verification / Alternative check:
Mutations in lacI can be complemented in trans (via plasmid), consistent with lacI being a separate gene product rather than part of the operon mRNA.

Why Other Options Are Wrong:

• O and P are core regulatory elements of the operon.
• “None of these” is false because one listed element (I) is indeed not part of the operon unit.

Common Pitfalls:
Equating “regulatory gene near an operon” with being part of it. Proximity does not define operon membership; shared transcription does.

Final Answer:
I (lacI, the repressor gene)