Final answer:
Bacterial RNA polymerase is essential for gene transcription in prokaryotes, with the sigma subunit providing specificity by recognizing promoter regions to initiate proper mRNA synthesis. Without it, the polymerase would bind randomly, producing nonfunctional mRNA.
Step-by-step explanation:
Bacterial RNA polymerase is a multi-subunit enzyme complex responsible for transcribing DNA into RNA in prokaryotic organisms such as E. coli. Comprising five polypeptide subunits, with two alpha (a) subunits, one beta (β) subunit, one beta prime (β') subunit, and the sigma (σ) subunit, it assembles into a core enzyme and a sigma factor inclusive holoenzyme. The core enzyme includes the two alpha subunits which assemble the polymerase on the DNA, the beta subunit which binds ribonucleoside triphosphates, and the beta prime subunit which binds the DNA template strand. The σ subunit's significance lies in its role during transcription initiation, where it contributes to transcriptional specificity by recognizing promoter regions, ensuring mRNA synthesis commences from appropriate initiation sites. Without the σ subunit, RNA polymerase would initiate transcription at random sites, leading to nonfunctional mRNA sequences and protein production.
The σ subunit is critical because it acts transiently, dissociating from RNA polymerase after the initiation of transcription. This selective binding and dissociation ensure that RNA polymerase properly engages with promoter sequences upstream of the transcription start site, a process vital for the regulation and fidelity of gene expression in bacterial cells.