Which subunit of bacterial RNA polymerase is responsible for the initiation of RNA synthesis?

The σ (sigma) subunit of bacterial RNA polymerase plays a crucial role in the initiation of RNA synthesis. It is responsible for the recognition and binding to specific promoter sequences on the DNA. The σ subunit helps the RNA polymerase core enzyme identify the correct sites to begin transcription, ensuring that RNA synthesis starts at the right location on the DNA template.

Once the σ factor binds to the promoter region, it facilitates the unwinding of DNA, allowing the core enzyme to access the single-stranded DNA template necessary for RNA synthesis. After the initiation phase is complete, the σ subunit typically dissociates from the RNA polymerase complex, allowing RNA synthesis to proceed with the core enzyme.

In contrast, the other subunits play different roles. The α subunit is involved in enzyme assembly and promoting interactions with regulatory proteins, while the β and β′ subunits form the catalytic core of the enzyme but are not directly responsible for promoter recognition in the initiation of RNA synthesis. Thus, the σ subunit is essential for guiding RNA polymerase to the appropriate starting point for transcription.