For a repressible operon to be transcribed, which of the following must be true?
a) a corepressor must be present.
b) rna polymerase and the active repressor must be present.
c) rna polymerase must bind to the promoter, and the repressor must be inactive.
d) rna polymerase cannot be present, and the repressor must be inactive.
e) rna polymerase must not occupy the promoter, and the repressor must be inactive.

A) A corepressor must be present

Explanation:

The key to operon expression lies in the sequence of the operator and the presence or absence of a repressor. When the repressor binds the operator, it prevents RNA  polymerase from initiating transcription. The capability of the  repressor to bind the operator and inhibit transcription depends on the conformation of the repressor, which is regulated  allosterically by a key compound in the metabolic pathway,  such as lactose or tryptophan. In a repressible operon, such as the tryptophan (or trp) operon, the repressor is unable to bind to the operator DNA by itself. Instead, the repressor is active as a  DNA-binding protein only when complexed with a specific  factor, such as tryptophan , which functions as  a corepressor.

When tryptophan is plentiful, tryptophan molecules act as corepressors by binding to the (1) inactive  repressor and (2) change its shape, allowing it to bind to the operator,  (3) preventing transcription of the structural genes

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