Answer:
d. regulatory; operator; regulatory
Explanation:
In prokaryotes, the regulation of gene expression is done at the transcription level by operons. Examples are the lac operon and the trp operon. To initiate transcription, RNA polymerase and a DNA segment are required for binding called a promoter.
DNA segments that are close to the promoter serve as binding sites for regulatory, activating or repressing proteins. They are called operons.
The Operon is a regulatory sequence close to prokaryotic DNA that lies between the structural genes and the promoter. It is responsible for controlling the transcription of structural genes. When the repressor, encoded by the regulatory gene, binds to the operator, the promoter is blocked which prevents transcription.
The Promoter is located next to an operator and is the site in the DNA molecule that binds to RNA polymerase (which is a regulatory protein) to initiate transcription.
