Final answer:
The RNA polymerase holoenzyme in E. coli includes a core enzyme consisting of two identical α subunits, one β subunit, and one β' subunit, plus the σ subunit, which is essential for transcription initiation.
Step-by-step explanation:
In E. coli, the RNA polymerase holoenzyme is composed of the core enzyme plus the σ subunit. The core enzyme is made up of four polypeptide subunits: two are identical α subunits, one β subunit, and one β' subunit. These subunits assemble to initiate the process of transcription, with each playing a vital role in the synthesis of mRNA from DNA. The two α subunits assemble the polymerase on the DNA, the β subunit binds to the ribonucleoside triphosphate, and the β' subunit binds the DNA template strand. The fifth subunit, σ, is crucial for transcription initiation, conferring specificity to ensure that transcription begins at the correct site.