Sigma factor: Difference between revisions

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== Specific Function and Structure ==

The '''σ-factor''' performs two chief functions: to direct the catalytic core of RNAP to the +1 start site of transcription, and finally to assist in the initiation of strand seperation of double-helical DNA, ultimately forming the transcription "bubble." Each gene promoter utilizes a specific promoter region about 40 bp upstream of the transcription start site, and therefore different σ-factors play a role in the regulation of different genes.

The '''σ-factor''' performs two chief functions: to direct the catalytic core of RNAP to the +1 start site of transcription, and finally to assist in the initiation of strand seperation of double-helical DNA, ultimately forming the transcription "bubble." Each gene promoter utilizes a specific promoter region about 40 bp upstream of the transcription start site, and therefore different σ-factors play a role in the regulation of different genes. This process, which includes association of the σ-factor with RNAP to recognize and open DNA at the promoter site, followed by dissociation of the σ to allow elongation, is referred to as the '''σ cycle'''.

===Domains===

~~Each~~ σ is composed of a variable number of structured domains. The simplest σ-factors have two domains, few have three, and others, called '''housekeeping σ-factors''' have 4 domains. Each of these domains has DNA-binding determinants, or motifs that recognize specific sequences and conformations in DNA. Most commonly, these recognized motifs occur at the -35 and -10 locations upstream of the +1 site. One such DNA binding motif, '''the helix-turn-helix motif''' <scene name='59/591940/Hth_motif/1'>HTH</scene>, helps specifically recognize DNA promoters at both the -35 and -10 positions. This HTH motif, used by most σ-factors, maintains its specificity and accuracy by binding in the '''major groove''' of DNA, where it can interact with the base pairs in the DNA double-helix. In many prokaryotes, these portions of DNA maintain consensus adenosine and thymine sequences, such as <scene name='59/591940/Ta_sequence/1'>TATAAT</scene>.

There are many types of σ-subunits, and each recognizes a unique promoter sequence. Furthmore, each σ is composed of a variable number of structured domains. The simplest σ-factors have two domains, few have three, and others, called '''housekeeping σ-factors''' have 4 domains. Each of these domains has DNA-binding determinants, or motifs that recognize specific sequences and conformations in DNA. Most commonly, these recognized motifs occur at the -35 and -10 locations upstream of the +1 site. One such DNA binding motif, '''the helix-turn-helix motif''' <scene name='59/591940/Hth_motif/1'>HTH</scene>, helps specifically recognize DNA promoters at both the -35 and -10 positions. This HTH motif, used by most σ-factors, maintains its specificity and accuracy by binding in the '''major groove''' of DNA, where it can interact with the base pairs in the DNA double-helix. In many prokaryotes, these portions of DNA maintain consensus adenosine and thymine sequences, such as <scene name='59/591940/Ta_sequence/1'>TATAAT</scene>.

===Restriction===

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@@ Line 7: / Line 7: @@
 == Specific Function and Structure ==
-The '''σ-factor''' performs two chief functions: to direct the catalytic core of RNAP to the +1 start site of transcription, and finally to assist in the initiation of strand seperation of double-helical DNA, ultimately forming the transcription "bubble."  Each gene promoter utilizes a specific promoter region about 40 bp upstream of the transcription start site,  and therefore different σ-factors play a role in the regulation of different genes.
+The '''σ-factor''' performs two chief functions: to direct the catalytic core of RNAP to the +1 start site of transcription, and finally to assist in the initiation of strand seperation of double-helical DNA, ultimately forming the transcription "bubble."  Each gene promoter utilizes a specific promoter region about 40 bp upstream of the transcription start site,  and therefore different σ-factors play a role in the regulation of different genes.  This process, which includes association of the σ-factor with RNAP to recognize and open DNA at the promoter site, followed by dissociation of the σ to allow elongation, is referred to as the '''σ cycle'''.
 ===Domains===
-Each σ is composed of a variable number of structured domains.  The simplest σ-factors have two domains, few have three, and others, called '''housekeeping σ-factors''' have 4 domains.  Each of these domains has DNA-binding determinants, or motifs that recognize specific sequences and conformations in DNA.  Most commonly, these recognized motifs occur at the -35 and -10 locations upstream of the +1 site.  One such DNA binding motif, '''the helix-turn-helix motif''' <scene name='59/591940/Hth_motif/1'>HTH</scene>, helps specifically recognize DNA promoters at both the -35 and -10 positions.  This HTH motif, used by most σ-factors, maintains its specificity and accuracy by binding in the '''major groove''' of DNA, where it can interact with the base pairs in the DNA double-helix.  In many prokaryotes, these portions of DNA maintain consensus adenosine and thymine sequences, such as <scene name='59/591940/Ta_sequence/1'>TATAAT</scene>.
+There are many types of σ-subunits, and each recognizes a unique promoter sequence.  Furthmore, each σ is composed of a variable number of structured domains.  The simplest σ-factors have two domains, few have three, and others, called '''housekeeping σ-factors''' have 4 domains.  Each of these domains has DNA-binding determinants, or motifs that recognize specific sequences and conformations in DNA.  Most commonly, these recognized motifs occur at the -35 and -10 locations upstream of the +1 site.  One such DNA binding motif, '''the helix-turn-helix motif''' <scene name='59/591940/Hth_motif/1'>HTH</scene>, helps specifically recognize DNA promoters at both the -35 and -10 positions.  This HTH motif, used by most σ-factors, maintains its specificity and accuracy by binding in the '''major groove''' of DNA, where it can interact with the base pairs in the DNA double-helix.  In many prokaryotes, these portions of DNA maintain consensus adenosine and thymine sequences, such as <scene name='59/591940/Ta_sequence/1'>TATAAT</scene>.
 ===Restriction===