The lac repressor and operon interaction in E. Coli, a perfect model for negative gene regulation in bacteria. In the protein’s active tetramer form, the lac repressor is bound to the promoter, prohibiting transcription of lac genes (the three genes LacZ, LacY, & LacA which encode for enzymes beta-galactosidase, permease, and transacetylase, respectively). Lacl also acts as a lever, prying open the minor groove of the DNA. The minor groove decreases in size with the bending of DNA when the full tetramer is bound.
The N terminal domains (where DNA binding to the major groove occurs) are shown, the palindromic DNA sequences, in red. each subunit containing a Helix-Turn-Helix motif. The repressor works synergistically with the Catabolite Activator Protein (CAP) in a protein-protein interaction. There are three operators, but only two operators are bound by the lac repressor at a time.
The core domains are shown in blue (where allolactose binds as an effector molecule).
Transcription of lac genes activation based on substrate ability (+ presence of, – absence of):
(+ glucose / + lactose) Operon off, CAP not bound (featured above)
(+ glucose  / – lactose) Operon off, lac repressor & CAP not bound
(– glucose /  – lactose) Operon off, lac repressor  not bound
(– glucose / + lactose) CAP & RNA pol bound, operon is on
(Image source: Matt Glassman, 1999)

The lac repressor and operon interaction in E. Coli, a perfect model for negative gene regulation in bacteria. 

In the protein’s active tetramer form, the lac repressor is bound to the promoter, prohibiting transcription of lac genes (the three genes LacZ, LacY, & LacA which encode for enzymes beta-galactosidase, permease, and transacetylase, respectively). Lacl also acts as a lever, prying open the minor groove of the DNA. The minor groove decreases in size with the bending of DNA when the full tetramer is bound.

The N terminal domains (where DNA binding to the major groove occurs) are shown, the palindromic DNA sequences, in red. each subunit containing a Helix-Turn-Helix motif. The repressor works synergistically with the Catabolite Activator Protein (CAP) in a protein-protein interaction. There are three operators, but only two operators are bound by the lac repressor at a time.

The core domains are shown in blue (where allolactose binds as an effector molecule).

Transcription of lac genes activation based on substrate ability (+ presence of, – absence of):

  • (+ glucose / + lactose) Operon off, CAP not bound (featured above)
  • (+ glucose  / – lactose) Operon off, lac repressor & CAP not bound
  • (glucose /  – lactose) Operon off, lac repressor  not bound
  • (– glucose / + lactose) CAP & RNA pol bound, operon is on

(Image source: Matt Glassman, 1999)

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Posted on Tuesday, 18 June
Tagged as: protein   gene transcription   gene regulation   molecular biology   bacteria   e coli   science  
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