Final answer:
The protein that assists RNA polymerase in prokaryotes to initiate transcription is called a sigma factor. This factor is essential for recognizing the promoter regions on DNA and helps form the holoenzyme necessary for transcription initiation.
Step-by-step explanation:
A protein that works with RNA polymerase in prokaryotes to initiate transcription is known as a sigma factor. In prokaryotic cells like E. coli, the RNA polymerase requires these sigma factors to recognize and bind to specific promoter sequences on the DNA, thereby starting the process of transcription. Once transcription has begun, the sigma factor disassociates from the RNA polymerase.
The holoenzyme is the complete form of prokaryotic RNA polymerase, consisting of subunits a, a, β, β', and σ, with the sigma factor (σ) being crucial for transcription initiation. Without this factor, the core enzyme would transcribe randomly and create non-functional mRNA molecules. The presence of sigma factors ensures that mRNA is synthesized from an appropriate initiation site.