Final answer:
The eukaryotic polymerase α subunit functions as a primer with low processivity, unlike the prokaryotic RNA polymerase holoenzyme which consists of five subunits including the essential σ subunit for initiation. Additional transcription factors are required for eukaryotic transcription.
Step-by-step explanation:
In eukaryotic cells, the polymerase α subunit functions as a primer, with this enzyme having low processivity, which means it detaches from the DNA template after synthesizing a short strand of RNA. In comparison, prokaryotic RNA polymerase, used by organisms such as E. coli, consists of five polypeptide subunits (denoted α, α, β, β', and σ) that come together to form the holoenzyme responsible for gene transcription. The σ subunit is crucial for transcription initiation as it ensures the polymerase attaches to the correct site on DNA, preventing random and unregulated synthesis of mRNA.
While prokaryotic polymerases can bind directly to DNA templates, eukaryotic transcription requires additional proteins known as transcription factors to initiate the process. This greater complexity in eukaryotic organisms is also reflected in the presence of three distinct RNA polymerases (α, β, and γ), each requiring a unique set of transcription factors to bind to the DNA template.