Final answer:
The sigma (σ) factor subunit of RNA polymerase provides specificity to promoter sequences in prokaryotes, ensuring correct initiation of transcription.
Step-by-step explanation:
The subunit of RNA polymerase II that helps to provide specificity for a particular promoter sequence is known as the σ factor (sigma factor) in prokaryotic organisms such as E. coli. The σ subunit of prokaryotic RNA polymerase recognizes consensus sequences found in the promoter region upstream of the transcription start site, allowing the RNA polymerase to initiate transcription at specific locations on the DNA. It is an essential component of the holoenzyme that ensures transcription begins from the appropriate initiation site, avoiding random transcription that would lead to nonfunctional mRNA.