RNA polymerase
Introduction
RNA polymerase is a type of enzyme that is essential in the process of transcription, where an RNA molecule is synthesized from a DNA template. This process is a key step in the overall process of gene expression, which allows the information stored in DNA to be used to produce proteins and other gene products.
Structure and Function
RNA polymerases are complex enzymes composed of multiple protein subunits. In bacteria, the core enzyme has five subunits: two α subunits, and one each of β, β', and ω. This core enzyme is capable of synthesizing RNA, but it requires a sigma factor to bind to specific DNA sequences and initiate transcription.
In eukaryotic cells, RNA polymerases are even more complex. There are three main types of RNA polymerase: RNA polymerase I, II, and III. Each of these has a different role in the cell and recognizes different sets of promoters, or DNA sequences that indicate where transcription should start.
RNA polymerase I is responsible for transcribing rRNA, which makes up the bulk of the RNA in the cell and is a key component of ribosomes, the cell's protein factories. RNA polymerase II transcribes mRNA, which carries the instructions for protein synthesis from the DNA to the ribosomes. RNA polymerase III transcribes tRNA, which carries amino acids to the ribosomes during protein synthesis, as well as other small RNAs.
Mechanism of Action
The process of transcription begins when RNA polymerase binds to a promoter sequence on the DNA. The enzyme then unwinds the DNA double helix to expose the template strand, which will be used to guide the synthesis of the new RNA molecule.
As RNA polymerase moves along the DNA, it adds nucleotides to the growing RNA chain. These nucleotides are complementary to the ones on the DNA template strand, so the sequence of the RNA molecule is a copy of the sequence of the DNA, with the exception that RNA contains the base uracil instead of thymine.
Once the RNA polymerase reaches a termination signal on the DNA, it releases the newly synthesized RNA molecule and detaches from the DNA. This new RNA molecule can then be processed and used in the cell.
Regulation of RNA Polymerase
The activity of RNA polymerase is tightly regulated to ensure that genes are expressed at the right time and in the right amount. In bacteria, the sigma factor plays a key role in this regulation by determining which genes the RNA polymerase can bind to and transcribe.
In eukaryotes, the regulation of RNA polymerase is more complex and involves a variety of transcription factors and other proteins. These factors can bind to the DNA near the promoter and influence the ability of RNA polymerase to initiate transcription. Some factors enhance transcription, while others inhibit it.
Role in Disease and Drug Target
Given its essential role in gene expression, it is not surprising that mutations in RNA polymerase or its regulatory factors can lead to disease. For example, some forms of cancer are associated with mutations that affect the activity of RNA polymerase II.
In addition, because of its crucial role in transcription, RNA polymerase is a target for certain antibiotics, such as rifampicin, which inhibits the bacterial RNA polymerase and thus prevents bacteria from expressing their genes and growing.