Final answer:
To initiate transcription in prokaryotes, the σ subunit of RNA polymerase binds to the promoter. After initiation, the σ subunit must dissociate for RNA polymerase to continue transcription.
Step-by-step explanation:
In prokaryotes, in order for the RNA polymerase to move forward with the transcription process, the σ subunit, also known as the sigma factor, must leave. The σ subunit's role is specifically to recognize consensus sequences in the promoter region upstream of the transcription start site.
This ensures that the RNA polymerase initiates transcription at the correct location. Once transcription initiation has occurred, the σ subunit dissociates from the RNA polymerase, allowing the core enzyme, which includes the subunits α (alpha), α (alpha), β (beta), and β' (beta-prime), to proceed with elongating the mRNA transcript.