Final answer:
The sigma subunit of bacterial RNA polymerase is required for promoter specificity and is involved in the initiation stage of transcription.
Step-by-step explanation:
The subunit of bacterial RNA polymerase (RNAP) that is required for promoter specificity is the sigma (σ) subunit. During bacterial transcription, the σ subunit of RNA polymerase is involved in the initiation stage. It recognizes consensus sequences found in the promoter region upstream of the transcription start site.
This subunit is critical because it ensures that the RNAP holoenzyme initiates transcription at the correct site on the DNA. Without the σ subunit, the core enzyme would transcribe from random sites, leading to nonsensical mRNA sequences which could result in nonfunctional proteins.