The Transcription Apparatus

Recall that, in replication, a number of different enzymes and proteins are required to bring about DNA synthesis. Although transcription might initially appear to be quite different, because a single enzyme—RNA polymerase— carries out all the required steps of transcription, on closer inspection, the processes are actually similar. The action of RNA polymerase is enhanced by a number of accessory proteins that join and leave the polymerase at different stages of the process. Each accessory protein is responsible for providing or regulating a special function. Thus, transcription, like replication, requires an array of proteins.

Bacterial RNA polymerase Bacterial cells typically possess only one type of RNA polymerase, which catalyzes the synthesis of all classes of bacterial RNA: mRNA, tRNA, and rRNA. Bacterial RNA polymerase is a large, multimeric enzyme (meaning that it consists of several polypeptide chains).

At the heart of bacterial RNA polymerase are four subunits (individual polypeptide chains) that make up the core enzyme: two copies of a subunit called alpha (a), a single copy of beta (p), and single copy of beta prime (p') (iFigure 13.10). The core enzyme catalyzes the elongation of the RNA molecule by the addition of RNA nu-cleotides. Other functional subunits join and leave the core enzyme at particular stages of the transcription process. The sigma (ct) factor controls the binding of the RNA polymerase to the promoter. Without sigma, RNA polymerase will initiate transcription at a random point along the DNA. After sigma has associated with the core enzyme (forming a holoenzyme), RNA polymerase binds stably only to the promoter region and initiates transcription at the proper start site. Sigma is required only for promoter binding and initiation; when a few RNA nucleotides have been joined together, sigma detaches from the core enzyme.

Many bacteria possess multiple types of sigma. E. coli, for example, possesses sigma 28 (ct28), sigma 32 (ct32), sigma 54 (ct54), and sigma 70 (ct70), named on the basis of their molecular weights. Each type of sigma initiates the binding of RNA polymerase to a particular set of promoters. For example, ct32 binds to promoters of genes that protect against environmental stress, ct54 binds to promoters of genes used during nitrogen starvation, and ct70 binds to many different promoters.

Other subunits provide the core RNA polymerase with additional functions. Rho (p) and NusA, for example, facilitate the termination of transcription.

Concepts 9

RNA is synthesized from ribonucleoside triphosphates. Transcription is 5': 3': each new nucleotide is joined to the 3'-OH group of the last nucleotide added to the growing RNA molecule. RNA synthesis does not require a primer.

Sigma factor

Core RNA polymerase a

RNA polymerase holoenzyme

Eukaryotic RNA polymerases

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