Transcription cycle

Figure 37-3. The transcription cycle in bacteria. Bacterial RNA transcription is described in four steps ...

Different RNAs are transcribed with different efficiencies, so that structural (transfer and ribosomal) RNAs are transcribed very efficiently. Each transcription cycle leads to a single molecule. In contrast,... 

The most detailed molecular information about the transcription cycle is available in bacterial systems. The synthesis of RNA is initiated at the promoter sequence by the enzyme RNA polymerase. A single RNA polymerase type is responsible for the synthesis of messenger, transfer, and ribosomal RNAs. 

The cr subunit is involved directly in promoter recognition. The complex lacking the polymerase core enzyme. To start transcription, the cr subunit directs the holoenzyme to a promoter site to form a binary complex in which there is a limited unwinding of the DNA duplex to generate an open promoter complex. This is the first step in the overall transcription cycle and is called template binding. 

Fig. 17-2 The transcription cycle in bacteria. P and T refer to promoter and termination sites, respectively, for a single RNA transcript, and pppA denotes the triphosphate ATP.

Fig. 9.12 A transcription cycle, according to P. R.Cook. A loop of chromatin is attached to the transcriptional machinery. The transcript is synthesized, while the template slides throigh the polymerase. When transcription is completed, the chromatin template detaches and a new cyle of transcription can start. (Based on Rg. 5 in ref. 44 and shown with permission of Science.)...

Transcription cycle of E. coli RNA polymerase showing dissociation of the ct subunit shortly after chain elongation begins, dissociation of the core enzyme during termination, and re-formation of the holoenzyme from the core enzyme and the a subunit. A previously joined core enzyme and a subunit will rarely become rejoined instead, reassociation occurs at random. 

Bacterial RNA polymerase. Most RNA polymerases (RNAPs) are large multisubunit proteins. However, bacterial viruses sometimes induce their own RNA polymerases, and these may be monomeric. For example, the 99-kDa (883-residue) phage T7-encoded polymerase is a single peptide chain with a structure and two-metal-ion active site resembling those of E. coli DNA polymerase 1.28 30a jg carry out all of the steps of the transcription cycle of the virus. In contrast, the most studied bacterial RNAP, that from E. coli, consists of five kinds of sub-units " with the composition ct2PP o(0. A similar composition has been foimd for RNAPs of other bacte-ria 33e Punctions of the five subunits can be correlated directly with components of archaeal and eukaryotic RNAPs. However, the latter contain additional subunits. The two a subunits in the E. coli enzyme have identical sequences, but their locations and interactions are different. ... 

Bacterial RNA polymerase is a multisubunit enzyme of 480,000. The four major subunits—15, 5, a, and O (M = 150,000 160,000 36,500 and 86,000, respectively)—are present in the ratio 1 1 2 1, and the total complex is more correctly called the RNA polymerase holoenzyme. The o subunit is involved directly in promoter recognition. The complex without the o subunit is called the RNA polymerase core enzyme. To start transcription, a-helices in the O subunit simultaneously contact base pairs in the -10 and -35 regions of the promoter. This directs the holoenzyme to the promoter site to form a binary complex in which there is a limited unwinding of the DNA duplex to generate an open promoter complex. This is the first step in the overall transcription cycle and is called template binding. The sequence of events in the transcription cycle is illustrated in Fig. 9-3. 

Vermeulen W, Houtsmuller AB (2002) The transcription cycle in vivo a blind watchmaker at work. Mol Cell 10 1264-1266... 

Kimura H, Sugaya K, Cook PR (2002) The transcription cycle of RNA polymerase II in living ceUs.J CeU Biol 159 777-782... 

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