Which Would Most Likely Be a Gene Under Constitutive Expression

Which would most likely be a gene under constitutive expression control?

A) A gene that encodes for a membrane transporter critical for uptake of a novel metabolite in the environment
B) A gene that encodes for a protein critical for metabolizing an environmental toxin
C) A gene that encodes for a protein that is produced in response to oxidative stress
D) A gene whose promoter is bound tightly by a transcriptional repressor
E) A gene that encodes for a portion of the ribosome