Final answer:
The repressor in a repressible operon blocks transcription in response to an abundance of an end product, while in an inducible operon, the repressor is deactivated by an inducer molecule to enable transcription.
Step-by-step explanation:
The fundamental difference between the repressor protein in a repressible operon and an inducible operon is the conditions under which they inhibit gene transcription. In a repressible operon, the repressor protein binds to the operator and blocks transcription when a specific end product (like tryptophan) is abundant.
Conversely, in an inducible operon, the repressor protein is bound to the operator and inhibits transcription by default, but when an inducer (such as lactose) is present, it binds to the repressor and prevents it from blocking transcription, thereby allowing gene expression. In contrast, in an inducible operon, the repressor protein is synthesized as an active protein that binds to the operator region of the operon by default, preventing gene transcription.