MBB 222 Lecture Notes - Lecture 28: Lac Operon, Lac Repressor, Camp Receptor Protein
20 Dec 2018
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Lecture 28 Part 4: Lac Operon
The two-fold symmetry axis suggested the repressor binds with two-fold symmetry.
The lac repressor binds DNA via a helix-turn-helix motif
The helix-turn-helix motif (HTH) is a common motif in DNA binding proteins. In this motif, two α-
helices are joined by a tight turn. The second of the two helices (the “recognition helix”) binds
in the DNA major groove where amino acid side chains contact the bases. The first helix
interacts mainly with the sugar-phosphate backbone.
Regulatory proteins often bind to DNA as dimers via helix-turn-helix motifs
Proteins that bind to DNA via the helix-turn-helix motif often bind as dimers with a two-fold
symmetry axis, to DNA that also has a two-fold symmetry axis: two adjacent binding sites that
are palindromic.
The lac operon is also subject to positive regulation
The lac operon has a weak (low efficiency) promoter. Even when allolactose (and lactose) is
present and the operon is derepressed, transcription doesn’t occur at high levels.
In addition to de-repression (induction), expression of the lac operon must be activated – this
occurs when glucose is *absent*.
cAMP receptor protein (CRP), also known as catabolite gene activator protein (CAP), is an
activator for the lac operon – a positive regulator. CRP binds to the effector cAMP, changing
its conformation and substantially increasing its affinity for the lac promoter. CRP-cAMP binding
to the lac promoter stimulates transcription by binding directly to RNA pol. CRP-cAMP
binding distorts the DNA, which may allow RNA polymerase to bind more effectively.
The lac operon is subject to positive regulation by CRP in the presence of cAMP
Document Summary
The two-fold symmetry axis suggested the repressor binds with two-fold symmetry. The lac repressor binds dna via a helix-turn-helix motif. The helix-turn-helix motif (hth) is a common motif in dna binding proteins. In this motif, two - helices are joined by a tight turn. The second of the two helices (the recognition helix ) binds in the dna major groove where amino acid side chains contact the bases. The first helix interacts mainly with the sugar-phosphate backbone. Regulatory proteins often bind to dna as dimers via helix-turn-helix motifs. Proteins that bind to dna via the helix-turn-helix motif often bind as dimers with a two-fold symmetry axis, to dna that also has a two-fold symmetry axis: two adjacent binding sites that are palindromic. The lac operon is also subject to positive regulation. The lac operon has a weak (low efficiency) promoter. Even when allolactose (and lactose) is present and the operon is derepressed, transcription doesn"t occur at high levels.
