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All of the genes in prokaryotes are transcribed by the same RNA polymerase. Two of the five polypeptide subunits that make up the polymerase in E. coli are identical. The polymerase core enzyme is made up of four of these subunits, identified by the letters, and ′. Every time a gene is transcribed, these subunits assemble; after transcription is finished, they disassemble.

The fifth subunit σ is only engaged in the beginning stages of transcription. By granting transcriptional specificity, the polymerase may start mRNA synthesis at the proper initiation location. Without it, the core enzyme would copy DNA from arbitrary locations, resulting in mRNA molecules that encoded protein nonsense. Therefore, it brings specificity.

Since the σ subunit of the E. coli polymerase confers specificity to transcription, other options a, b, and c are irrelevant due to the following reason,

• To assemble the polymerase on the DNA, two α- -subunits are required.
• The β -subunit attaches to the ribonucleoside triphosphate that will eventually become a component of the developing, or "recently born" mRNA molecule,
• while the β' subunit joins the DNA template strand.