Inducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymes

An E. coli cell growing on a carbon source such as glucose or glycerol contains fewer than 10 molecules of β -galactosidase. In contrast, the same cell contains several thousand molecules of the enzyme when grown on lactose. The presence of lactose in the culture medium induces a large increase in the amount of β -galactosidase by eliciting the synthesis of new enzyme molecules rather than by activating a preexisting but inactive precursor. A crucial clue to the mechanism of gene regulation is that two other proteins are synthesized in concert with β -galactosidase namely, galactoside permease and thiogalactoside transacetylase . The permease is required for the transport of lactose across the bacterial cell membrane. The transacetylase is not essential for lactose metabolism but appears to play a role in the detoxification of compounds that also may be transported by the permease. Thus, the expression levels of a set of enzymes that all contribute to the adaptation to a given change in the environment change together . Such a coordinated unit of gene expression is called an operon .

Although repressors are free to diffuse through the cell, how does — for example — the lac repressor find the single stretch of 24 base pairs of the operator out of the million base pairs of DNA in the E. coli genome ? It turns out the repressor is free to bind anywhere on the DNA using both

  • hydrogen bonds and
  • ionic (electrostatic) interactions between its positively-charged amino acids ( Lys, Arg ) and the negative charges on the deoxyribose-phosphate backbone of the DNA.
Once astride the DNA, the repressor can move along it until it encounters the operator sequence. Now an allosteric change in the tertiary structure of the protein allows the same amino acids to establish bonds — mostly hydrogen bonds and hydrophobic interactions — with particular bases in the operator sequence.

Inducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymes

inducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymes

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inducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymesinducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymesinducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymesinducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymesinducible operons usually encode anabolic enzymes and repressible operons encode catabolic enzymes

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