E. coli RNA polymerase consists of α, β, β′, ω, and σ subunits. Transcription is initiated by the binding of σ to promoter sequences. After synthesis of about the first ten nucleotides of RNA, the core polymerase dissociates from σ and travels along the template DNA as it elongates the RNA chain. Transcription then continues until the polymerase encounters a termination signal.
Eukaryotic cells contain three distinct nuclear RNA polymerases that transcribe genes encoding mRNAs, miRNAs, and lncRNAs (polymerase II), rRNAs (polymerases I and III), and tRNAs (polymerase III). Rather than binding directly to promoter sequences, eukaryotic RNA polymerases require additional proteins (general transcription factors) to initiate transcription. The recruitment of RNA polymerase II to promoters requires a minimum of five general transcription factors. Other factors are required for RNA polymerases I and III to bind their promoters.
rRNAs and tRNAs are derived by cleavage of long primary transcripts in both bacteria and eukaryotic cells. rRNAs are modified by methylation and pseudouridine formation, and various bases are modified in tRNAs. Eukaryotic pre-mRNAs are modified by the addition of 5′ 7-methylguanosine caps and 3′ poly-A tails, in addition to the removal of introns by splicing, which takes place in large complexes, called spliceosomes, composed of proteins and snRNAs. The snRNAs recognize sequences at the splice sites of pre-mRNAs and catalyze the splicing reaction. Exons can be joined in various combinations as a result of alternative splicing, which provides an important mechanism for tissue-specific control of gene expression. The sequences of mRNAs can also be modified by RNA editing, in mammalian cells involving either the deamination of cytosine to uridine or of adenosine to inosine. mRNAs in eukaryotic cells are degraded at different rates, providing an additional mechanism for control of gene expression.