Termination and antitermination: RNA polymerase runs a stop sign

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Abstract
The transcription elongation complex is composed of a multisubunit RNA polymerase (RNAP), a DNA template and a nascent RNA. This complex is normally extremely stable but can be efficiently disassembled by the action of select proteins, called termination factors, or specific sequences, called intrinsic terminators.Regulated transcription termination is the underlying mechanism of attenuation, riboswitch function and polar repression of downstream expression. Efficient termination is also necessary to halt transcription elongation complexes at the end of genes in order to block unproductive or aberrant expression of downstream genes.Expression of many genes, most notably those encoding ribosomal RNA, is critically dependent on antitermination mechanisms that modify RNAP into a termination-resistant state. Antitermination factors can modulate or block the activity of termination factors, override intrinsic terminators in leader sequences under conditions necessitating downstream expression, and modify the activity of RNAP to provide expression of select genes (for example, upon phage infection). Many antitermination factors act just once during mRNA synthesis by blocking the formation of an RNA hairpin structure that is necessary for intrinsic termination. These factors usually employ direct binding to RNA, and structures as large as translating ribosomes are used to determine the RNA fold. Processive antitermination factors bind to, and modify the activity of, elongating RNAP such that the modified complex is resistant to multiple or tandem downstream termination signals. Studies of antitermination factors provide important insights into mechanisms of processive RNA synthesis in all domains. These factors provide stability to the transcription complex, either by shielding the encapsulated nucleic acids or preventing structural changes in RNAP necessary for RNA release and complex disassembly.

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