Question

consider this diagram of an operon.
suppose a protein binds to the red region and prevents rna polymerase from moving past that region. how would this protein most likely impact transcription of the genes in the operon?
transcription of structural sequence 1, closest to the red region, would be inhibited. structural sequences 2 through 4 would be transcribed normally.
transcription of structural sequence 1, closest to the red region, would increase. transcription of structural sequences 2 through 4 would be inhibited.
the transcription of structural sequences 1 through 4 would be inhibited.
the transcription of structural sequences 1 through 4 would increase.

Explanation:

In an operon, structural genes are transcribed together as a single mRNA (polycistronic mRNA) because they share a common promoter. If a protein binds to the red region (downstream of the promoter but upstream of structural sequence 1) and blocks RNA polymerase from moving past, RNA polymerase can't transcribe any of the structural sequences (1 - 4) since transcription proceeds from the promoter through the structural genes. So all structural sequences 1 - 4 will have inhibited transcription. Let's analyze the options:

• First option: Incorrect, as structural genes in an operon are transcribed together, so 2 - 4 can't be transcribed normally if 1 is inhibited.
• Second option: Incorrect, binding to block RNA polymerase won't increase transcription of sequence 1.
• Third option: Correct, since RNA polymerase can't move past the red region, it can't transcribe any of the structural sequences 1 - 4.
• Fourth option: Incorrect, blocking RNA polymerase would inhibit, not increase, transcription.

Answer:

C. The transcription of structural sequences 1 through 4 would be inhibited.