The mmm operon, which has sequences A$,$ B$,$ C$,$ and D $($which may be

structural genes or regulatory sequences$),$ encodes enzymes $1$ and $2.$ Mutations in sequences A$,$ B$,$ C$,$ and D have the following effects, where a plus sign $(+)$ indicates that the enzyme is synthesized and a minus sign $()$ indicates that the enzyme is not synthesized.

mmm absent mmm present

Mutation in Enzyme Enzyme Enzyme Enzyme sequence $121$

No mutation $++$ A$+$ B$+++$ C$+$ D$$

a$.$ Is the mmm operon inducible or repressible?

b$.$ Indicate which sequence $($A$,$ B$,$ C$,$ or D$)$ is part of the following components of the operon:

$24.$

Regulator gene $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ Promoter $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ Structural gene for enzyme $1\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ Structural gene for enzyme $2\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$