Transcription - Transcription An Overview In all species...

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Transcription: An Overview In all species, transcription begins with the binding of the RNA polymerase complex (or holoenzyme) to a special DNA sequence at the beginning of the gene known as the promoter. Activation of the RNA polymerase complex enables transcription initiation, and this is followed by elongation of the transcript. In turn, transcript elongation leads to clearing of the promoter, and the transcription process can begin yet again. Transcription can thus be regulated at two levels: the promoter level (cis regulation) and the polymerase level (trans regulation). These elements differ among bacteria and eukaryotes. Transcription in Bacteria In bacteria, all transcription is performed by a single type of RNA polymerase. This polymerase contains four catalytic subunits and a single regulatory subunit known as sigma (s). Interestingly, several distinct sigma factors have been identified, and each of these oversees transcription of a unique set of genes. Sigma factors are thus discriminatory, as each binds a distinct set of promoter sequences. A striking example of the specialization of sigma factors for different gene promoters is provided by bacterial sporulation in the species Bacillus subtilis . This bacterium exists in two states: vegetative (growing) and sporulating. Genes involved in spore formation are not normally expressed during vegetative growth. Remarkably, expression of a gene encoding a novel sigma factor turns on the first genes for sporulation. Subsequent expression of different sigma factors then turns on new sets of genes needed later in the sporulation process (Losick & Stragier, 1992). Each of these sigma factors recognizes the promoters of the genes in its group, not those "seen" by other sigma factors. This simple example illustrates how transcription can be regulated in both cis and trans to cause changes in cell function. Therefore, while bacteria accomplish transcription of all genes using a single kind of RNA polymerase, the use of different sigma factor subunits provides an extra level of control. Transcription in Eukaryotes
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"Christmas tree-like" structures can be visualized during active transcription. Yeast strains conditionally expressing either the U3 snoRNA or Utp7 from a galactose promoter were used to make the chromatin spreads. Copyright 2002 Nature Publishing Group. Dragon, F. et al. Nature 417, 967-970. Eukaryotic cells are more complex than bacteria in many ways, including in terms of transcription. Specifically, in eukaryotes, transcription is achieved by three different types of RNA polymerase (RNA pol I-III). These polymerases differ in the number and type of subunits they contain, as well as the class of RNAs they transcribe; that is, RNA pol I transcribes ribosomal RNAs (rRNAs), RNA pol II transcribes RNAs that will become messenger RNAs (mRNAs) and also small regulatory RNAs, and RNA pol III transcribes small RNAs such as transfer RNAs (tRNAs). Because RNA pol II transcribes protein-encoding genes, it has been of particular
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Transcription - Transcription An Overview In all species...

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