Transcription converts genetic information from a DNA template into RNA, serving as the initial step in gene expression. While its core purpose is consistent across all organisms, the specific mechanisms vary significantly between eukaryotes and bacteria. These differences reflect the distinct cellular complexities and regulatory needs of these two domains of life.
Foundational Differences
A primary distinction in transcription lies in its cellular location. In eukaryotes, transcription occurs within the nucleus, a membrane-bound compartment. The resulting RNA must then be transported to the cytoplasm for protein synthesis. In contrast, bacterial transcription takes place in the cytoplasm, where genetic material is not enclosed by a membrane.
RNA polymerases, the enzymes responsible for transcription, also differ. Eukaryotic cells possess multiple distinct RNA polymerases, each specialized for transcribing different types of RNA: RNA polymerase I for ribosomal RNA (rRNA), RNA polymerase II for messenger RNA (mRNA) and some small RNAs, and RNA polymerase III for transfer RNA (tRNA) and other small RNAs. Bacteria, however, typically have a single RNA polymerase that synthesizes all classes of RNA.
The DNA organization further contributes to these differences. Eukaryotic DNA is packaged into chromatin, with strands wrapped around histones to form nucleosomes. This compact structure influences gene accessibility, requiring chromatin remodeling to expose DNA for transcription. Bacterial DNA is generally less complex, existing as a relatively naked, circular molecule in the cytoplasm.
Initiating Transcription
Transcription initiation involves specific DNA sequences called promoters, which serve as RNA polymerase binding sites. Bacterial promoter regions typically contain two conserved sequences: the -10 region (Pribnow box) and the -35 region, located approximately 10 and 35 nucleotides upstream from the transcription start site. These are recognized by the bacterial RNA polymerase’s sigma (σ) factor.
Eukaryotic promoters are more intricate, often featuring a core promoter element like the TATA box, found about 25-30 nucleotides upstream of the transcription start site. Beyond this core, eukaryotes also utilize regulatory elements such as enhancers and silencers, which can be located far from the gene. Recruitment of RNA polymerase II to eukaryotic promoters requires general transcription factors (GTFs). These GTFs assemble at the promoter to form a pre-initiation complex, facilitating RNA polymerase binding and positioning.
The sigma factor is essential for bacterial RNA polymerase to accurately locate and bind to the promoter. Once transcription begins, it often dissociates, allowing the core enzyme to proceed with elongation. In eukaryotes, RNA polymerase interacts with DNA via general and specific transcription factors, which regulate gene expression.
Elongation and Termination
Once initiated, elongation, where RNA polymerase synthesizes a complementary RNA strand from the DNA template, shares fundamental similarities in both eukaryotes and bacteria. The enzyme moves along the DNA, unwinding the double helix and adding ribonucleotides to the growing RNA chain. Eukaryotic elongation rates are typically slower, however, and must contend with complex chromatin structure, often requiring continuous modification.
Transcription termination mechanisms are distinct between the two domains. Bacteria use two primary mechanisms: Rho-dependent and Rho-independent (intrinsic) termination. Rho-independent termination relies on specific RNA transcript sequences that form a stable hairpin loop, followed by uracil residues. This hairpin causes RNA polymerase to pause, and weak bonds between the uracil-rich RNA and DNA template lead to RNA transcript dissociation.
Rho-dependent termination involves the Rho factor, an RNA helicase. The Rho protein binds to specific sequences on the nascent RNA, moves along the transcript, and eventually catches up to the paused RNA polymerase. Its helicase activity then unwinds the RNA-DNA hybrid, releasing the RNA polymerase and terminating transcription.
In eukaryotes, RNA polymerase II typically transcribes past the gene’s actual end. Termination is often linked to a polyadenylation signal sequence in the nascent RNA, triggering transcript cleavage and subsequent polymerase dissociation.
RNA Maturation and Translation Link
A significant difference between eukaryotic and bacterial gene expression lies in RNA processing after transcription and its link to translation. Eukaryotic messenger RNA (mRNA) undergoes extensive post-transcriptional modifications within the nucleus before translation.
These modifications include adding a 7-methylguanosine cap to the mRNA’s 5′ end. This 5′ cap protects mRNA from degradation, facilitates its nuclear transport, and initiates protein synthesis.
Splicing is another key eukaryotic modification, where non-coding introns are removed from pre-mRNA, and coding exons are ligated together. This allows for alternative splicing, where different exon combinations can produce multiple protein variants from a single gene.
Additionally, a poly-A tail (a stretch of adenine nucleotides) is added to the mRNA’s 3′ end. This tail contributes to mRNA stability, nuclear export, and translation initiation.
In contrast, bacterial mRNA generally undergoes minimal post-transcriptional modification. It is often functional immediately after transcription, lacking the 5′ cap, poly-A tail, and intron splicing characteristic of eukaryotes. While some bacterial RNAs show modifications, they are not as widespread or complex as those in eukaryotes.
The physical separation of transcription and translation in eukaryotes, due to the nuclear envelope, means these processes are uncoupled. Transcription must complete in the nucleus, and mature mRNA then exports to the cytoplasm before ribosomes begin protein synthesis. This uncoupling provides additional opportunities for gene regulation.
In bacteria, however, transcription and translation are often coupled, occurring simultaneously in the cytoplasm. Ribosomes can attach to nascent mRNA and begin translating it into protein even before the entire gene is transcribed. This coupling allows for rapid response to environmental changes and efficient protein production.