Final answer:
The specificity in starting transcription in E. coli is conferred by the sigma (σ) subunit of the RNA polymerase. It is crucial for initiation, targeting the polymerase to the right section of DNA to begin mRNA synthesis.
Step-by-step explanation:
The subunit of the E. coli RNA polymerase that confers specificity to transcription is the sigma (σ) subunit. This is the fifth subunit involved only in transcription initiation. The sigma subunit is essential for correctly identifying the start sites for RNA synthesis, ensuring that messenger RNA (mRNA) is transcribed from the proper initiation site.
Without the sigma factor, the core enzyme would start transcription at random places on the DNA, leading to the production of non-functional mRNA. Therefore, during the stage of bacterial transcription known as initiation, the sigma subunit plays a vital role. The RNA polymerase consisting of all five subunits including sigma is referred to as the holoenzyme.