RNA polymerase is an enzyme responsible for transcription, the initial step in gene expression. This multi-subunit enzyme synthesizes RNA molecules using a DNA template, converting genetic information from DNA into RNA. It initiates, elongates, and terminates RNA chains, which is necessary for regulating gene expression and ensuring accurate genetic information flow within a cell.
RNA Polymerase and Its Accuracy
RNA polymerase does not possess the same extensive proofreading mechanisms found in DNA polymerase. Its error rate is approximately one error per 100,000 nucleotides incorporated, which is significantly higher than the error rate of DNA polymerase. DNA polymerase, for instance, has an error rate of one mutation per 10^8 to 10^10 bases per generation. This highlights a lower fidelity in transcription compared to DNA replication.
RNA polymerase exhibits some proofreading activities, such as pyrophosphorolysis, but these are not as robust as the 3′ to 5′ exonuclease activity of DNA polymerase. The higher error rate of RNA polymerase means transcription errors can occur, leading to alterations in the RNA sequence. However, these errors are transient and do not permanently alter the cell’s genetic blueprint.
Why RNA Errors Are Not Permanent
The higher error rate of RNA polymerases does not pose a significant problem for the cell due to two primary reasons. First, most RNA molecules, such as messenger RNA (mRNA), are transient and have a short lifespan. For instance, bacterial mRNAs often have half-lives measured in minutes, while mammalian mRNAs can have half-lives of around 10 hours. This transient nature means that any errors are quickly degraded, preventing the accumulation of faulty molecules.
Second, errors in RNA do not permanently alter the genetic code stored in DNA. DNA serves as the “master copy” of genetic information, and it remains unchanged even if errors occur during RNA synthesis. If a faulty RNA molecule is produced, it can be degraded and replaced by a new, correctly synthesized RNA molecule from the original DNA template. This ensures the cell’s long-term genetic integrity.
Specific RNA Polymerases That Proofread
While most RNA polymerases lack robust proofreading, some specific types do exhibit proofreading activity. RNA polymerase III (RNA Pol III) is one such example. This enzyme synthesizes stable and highly abundant non-coding RNAs, such as transfer RNA (tRNA) and ribosomal RNA (rRNA), which are essential for protein synthesis and other cellular processes.
The high fidelity of RNA Pol III is important because errors in these stable RNA molecules could have lasting cellular consequences. RNA Pol III possesses an efficient proofreading activity that can increase its transcriptional fidelity by a factor of 1000 over the error rate determined solely by selectivity. This proofreading mechanism involves the enzyme slowing synthesis after a misincorporation and then cleaving off the mismatched base, often through nucleolytic cleavage at the same active site used for polymerization.