Gene Expression

Negative regulatory system- A gene repressor binds to a silencer to prevent gene expression.

Positive regulatory system- A gene activator binds to an enhancer to encourage gene expression.

Silencers and enhancers are particular bits of code to which activators and repressors bind.

If expression increases- the gene is induced (positively or negatively)

If expression decreases- the gene is repressed (positively or negatively)

A gene contains:

       Shine dalgarno sequence
       Operon-silencer or enhancer
       promoter
       cistron-reading frame
      terminator (probably rho dependent)

Note: There is NO WAY to turn a gene off completely in bacteria

Lac Operon-

3 Genes

LacI (not in the lac operon!!! Contains lac repressor)

LacZ(Beta galactosidase)----changes lactose into allolactose---which binds to the repressor on lacI to induce the lac operon

LacY(lactose permease) Lets lactose into the cell

LacA(galactosidase transacetylase) Vestigial perhaps

When repressing a gene-this prevents the sigma factor from finding the promoter, and then the gene is not expressed.