An operon is controlled by a repressor. when the repressor binds to a small molecule, it binds to dna near the operon. the operon is constitutively expressed if a mutation prevents the repressor from binding to the small molecule. the type of control illustrated . negative inducible. b. negative repressible. c. positive inducible. d. positive repressible. e. catabolite repression.

b. negative repressible

Explanation:

A repressible operon is an operon that is normally active but is negatively controlled by a repressor protein. Binding of the repressor protein to the specific site on DNA prevents the expression of genes of operon. Since the repressor protein prevents the expression of the operon, it exhibits negative regulation upon the operon.

The repressor becomes active when it binds to a corepressor. The active repressor binds to the operator sequence, one of the control sequence of the operon. Binding of repressor to the operator makes the operon transcriptionally inactive. Any mutation that does not allow the repressor protein to bind to the operator region would result in constitutive expression of the operon.  

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