Changes

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::Bacterial RNA polymerases are composed of multiple subunits.  The core enzyme consists of 2 alpha, beta and beta prime subunits.  This enzyme contains the catalytic activity for RNA synthesis.  Association of the core enzyme with the sigma subunit is required for initiation of transcription.  This complex is referred to as the holoenzyme.  The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence.  In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35.  The sigma subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site).  This complex is referred to as the closed complex.

::Bacterial RNA polymerases are composed of multiple subunits.  The core enzyme consists of 2 alpha, β and β′ subunits.  This enzyme contains the catalytic activity for RNA synthesis.  Association of the core enzyme with the sigma subunit is required for initiation of transcription.  This complex is referred to as the holoenzyme.  The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence.  In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35.  The sigma subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site).  This complex is referred to as the closed complex.

::The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription.  This is referred to as the open complex.

::The second step in transcription initiation involves the unwinding of the DNA between the -10 region and the start site of transcription.  This is referred to as the open complex.

[[Image:Image010.gif|left|thumb|Termination of Prokaryotic Transcription]]

[[Image:Image010.gif|left|thumb|Termination of Prokaryotic Transcription]]

===Eukaryotic Transcription===

===Eukaryotic Transcription===

 

::The basic features of RNA synthesis are shared between prokaryotes and eukaryotes; however, transcription in eukaryotes differs in that it is significantly more complex. First, rather than having a single RNA polymerase, eukaryotes have three different RNA polymerases, each of which transcribes a different set of genes. RNA polymerase I transcribes three types of rRNA (the 18S, 5.8S, and 28S species), RNA polymerase II transcribes mRNA, and RNA polymerase III transcribes tRNA and the smallest rRNA (the 5S species). The eukaryotic RNA polymerases consist of between eight and fourteen subunits, with two of them corresponding to the β and β′ subunits of prokaryotic RNA polymerases.

::In eukaryotes, there are three types of polymerases, each of which transcribes different subsets of RNAs. RNA Polymerase I is responsible for the synthesis of rRNA (a component of the ribosomes) and RNA Polymerase III is responsible for the synthesis of tRNAs and other small stable RNAs.  We will focus here on RNA Polymerase II (Pol II), which is responsible for the synthesis of all mRNAs (protein encoding RNAs).

::All three eukaryotic polymerases have some homology to the E. coli core polymerase (alpha2betabetaprime), but also contain additional subunits not found in prokaryotes.  In total, approximately 12-17 subunits (varies with the species) are required for activity in vivo, making eukaryotic polymerases much more complex than prokaryotic polymerases.

::All three eukaryotic polymerases have some homology to the E. coli core polymerase (alpha2betabetaprime), but also contain additional subunits not found in prokaryotes.  In total, approximately 12-17 subunits (varies with the species) are required for activity in vivo, making eukaryotic polymerases much more complex than prokaryotic polymerases.

==Eukaryotic RNA Processing==

==Eukaryotic RNA Processing==