Final answer:
The σ subunit is added to the bacterial RNA Polymerase core enzyme consisting of subunits α, α, ß, and ß', to form the holoenzyme, which is responsible for transcription initiation.
Step-by-step explanation:
In bacterial systems like E. coli, the RNA Polymerase core enzyme consists of four subunits: α, α, ß, and ß'. These are responsible for the elongation of the mRNA strand during transcription. However, for the initiation of transcription, a fifth subunit, σ (σ), is required. The σ subunit is crucial for the RNA Polymerase to recognize and bind to specific promoter regions of DNA, ensuring that transcription begins at the correct site. Without the σ subunit, the core enzyme would lack specificity, leading to random initiation and non-functional mRNA.
Therefore, the holoenzyme, which includes all five subunits (α, α, ß, ß', and σ), is responsible for transcription initiation and is essential for producing accurate and functional mRNA transcripts.