Question

The sigma subunit of bacterial RNA polymerase _____.

1) binds to a bacterial gene's promoter
2) is composed of both polypeptide and RNA molecules
3) is required for RNA polymerization
4) is required for termination of transcription
5) is required for ribosomal binding

Answer 1

The sigma subunit of bacterial RNA polymerase binds to the promoter region of a gene to initiate transcription. It dissociates from the RNA polymerase after initiation, and is not involved in polymerization, termination, or ribosomal binding.

Step-by-step explanation:

The sigma subunit of bacterial RNA polymerase binds to a bacterial gene's promoter. This is its primary function during the initiation of transcription. The sigma subunit (σ) is a component of the prokaryotic RNA polymerase holoenzyme, and it is crucial for starting the transcription process by recognizing specific promoter regions on the DNA. It has no role in RNA polymerization, termination of transcription, ribosomal binding, or being composed of RNA molecules.

In E. coli, RNA polymerase consists of five polypeptides: two α-subunits, one β-subunit, one β'-subunit, and the σ-subunit. The σ-subunit is exclusively involved in transcription initiation, providing specificity to ensure mRNA synthesis begins at the correct initiation site. After initiation, the σ-subunit dissociates from the rest of the enzyme, which then continues the elongation phase of transcription without it.