negative regulatory proteins bind to operator sequences in the dna and prevent or weaken rna...

Negative regulatory proteins bind to operator sequencesin the DNA and prevent or weaken RNA polymerase binding

Most prokaryotic mRNA molecules are polycistronic, theyencode multiple genes. These genes are usually involved in the same biochemical event.

A single promoter controls the expression of these genes.

This functional unit of DNA is called an operon.

A classical example oftranscriptional regulation

is lactose metabolismin E. coli.

Proteins required for lactose metabolism inE. coli are encoded by

the lac operon.

The E. coli lac operon

lacI – encodes the Lac repressor proteinlacZ – encodes b-galactosidaselacY – encodes galactose permeaselacA – encodes transacetylaseO2 and O3 are pseudooperators

The Lac repressor protein is thought to bind to the main operatorand one of the pseudooperators, forming a loop in the DNA.

When lactose is present in high concentrations, the lactose metabolismgene products are needed in a cell. In the absence of lactose, the Lac repressor protein binds to the operator in the DNA, repressingtranscription. The Lac repressor, however, binds to allolactose, ametabolite of lactose, inducing a conformational change that abolishesbinding to the DNA operator sequence. Transcription is no longer repressed.

- allolactose transparent+ allolactose bold

DNA binding proteins contain amino acids that hydrogen bond tofunctional groups in the major groove of DNA.

DNA sequences recognized by regulatory proteins are often inverted repeats of a short DNA sequence. These repeats form a palindrome with two-fold symmetry about a central axis. Regulatory proteins are often dimeric. Each subunit binds to one strand of the DNA.

5’-TACGGTACTGTGCTCGAGCACTGCTGTACT-3’3’-ATGCCATGACACGAGCTCGTGACGACATGA-5’

central axis

The Lac repressor protein

The Lac repressor is a tetramer of four identicalprotein subunits.

There are DNA-bindingdomains on each subunitshown in blue.

The allolactose binding domain (green) is connectedto the DNA binding domain through linker helices (yellow).

Tetramerization domains (red)form contacts between subunits.

The Lac repressor protein

The Lac repressor is a tetramer of four identicalprotein subunits.

There are DNA-bindingdomains on each subunitshown in blue.

The allolactose binding domain (green) is connectedto the DNA binding domain through linker helices (yellow).

Tetramerization domains (red)form contacts between subunits.

The DNA binding domains of the Lac repressor contain a helix-turn-helix motif, a structure critical for the interaction of many

proteins with DNA.

helix

turnhelix

Lac repressor protein

(lacI)

Lac repressor bound to DNA

Lac repressor bound to DNA