- Bacterial RNA polymerase is a multi-subunit enzyme complex responsible for transcription, the process of synthesizing RNA from a DNA template. This enzyme plays a central role in gene expression by catalyzing the formation of an RNA strand complementary to one of the DNA strands.
- Unlike eukaryotes, which use three distinct RNA polymerases (Pol I, II, and III) for different classes of RNA, bacteria rely on a single type of RNA polymerase to transcribe all types of RNA, including messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).
- The core bacterial RNA polymerase is composed of five subunits: two alpha (α) subunits, one beta (β), one beta prime (β′), and one omega (ω) subunit, forming the core enzyme (α₂ββ′ω). This core enzyme possesses the catalytic activity needed for RNA synthesis but cannot independently initiate transcription at specific promoters. For promoter recognition, the core enzyme associates with a sigma (σ) factor, forming the holoenzyme (α₂ββ′ωσ). The sigma factor directs the holoenzyme to specific DNA sequences known as promoters, enabling the polymerase to initiate transcription at the correct site.
- Promoters typically contain conserved motifs such as the -10 (Pribnow box) and -35 regions, which are recognized by the sigma factor. Upon binding to the promoter, the RNA polymerase unwinds the DNA to form a transcription bubble, allowing the enzyme to begin synthesizing RNA. Once transcription is initiated and the RNA chain reaches about 10 nucleotides in length, the sigma factor often dissociates, allowing the core polymerase to continue elongating the RNA chain.
- During elongation, RNA polymerase moves along the DNA, adding ribonucleotides complementary to the DNA template strand. The enzyme has intrinsic proofreading ability, allowing it to correct occasional errors by backtracking and cleaving off misincorporated nucleotides. This contributes to the fidelity of RNA synthesis, although bacterial RNA polymerase is generally less accurate than DNA polymerase.
- Transcription in bacteria ends through one of two mechanisms: Rho-independent (intrinsic) termination or Rho-dependent termination. In the intrinsic mechanism, transcription is halted by the formation of a GC-rich hairpin loop in the RNA followed by a series of uracils, which destabilizes the RNA-DNA hybrid and causes the polymerase to dissociate. In the Rho-dependent pathway, the Rho protein, an RNA helicase, binds to the nascent RNA and travels along it until it catches up with the polymerase, inducing dissociation.
- The activity of bacterial RNA polymerase is highly regulated, enabling bacteria to respond quickly to environmental changes. Different sigma factors can be used under various stress conditions or growth phases to redirect RNA polymerase to distinct sets of genes. For instance, σ⁷⁰ is the primary sigma factor for housekeeping genes, while σ³² is involved in heat shock response. Additionally, proteins like transcription factors, repressors, and activators modulate RNA polymerase activity by enhancing or inhibiting promoter binding and initiation.
- Bacterial RNA polymerase is also a key target for antibiotics. Drugs such as rifampicin bind to the β subunit and inhibit transcription initiation, making RNA polymerase an effective target in the treatment of bacterial infections like tuberculosis. Resistance to such antibiotics often arises from mutations in the rpoB gene, which encodes the β subunit of the polymerase.
