Chapter 15

Regulation of gene expression

bacterial gene expression regulation

replication, recombination, repair, cell division

inducible enzymes

produced only when specific substrates are present for the bacteria to adapt to the environment

constitutive enzymes

continuously produced regardless of the chemical makeup of the environment

repressible system

abundance of a molecule inhibits gene expression, involves molecules that are end products of anabolic biosynthetic pathways, conserves energy, ex. trp operon

negative control

genetic expression occurs unless shut off by a regulator molecule

positive control

transcription occurs only when a regulator molecule directly stimulates RNA production

operons

genes coding for enzymes regulated by a single regulatory region, mostly located upstream of the operon

cis-acting

regulatory region on the same strand as the genes

trans-acting

binding at the cis-acting sites regulate the gene cluster negatively or positively

what is lactose broken into?

galactose and glucose

structural genes

genes coding for primary structure of enzyme (lacZ, lacY, lacA in lac operon transcribed as one polycistronic mRNA)

lacZ

encodes b-galactosidase, which converts lactose (disaccharide) to the products (monosaccharides)

lacY

encodes permease, which facilitates the entry of lactose into the bacterial cell

lacA

encodes transacetylase, which removes toxic by products of lactose digestion

gratuitous inducers

chemical analogs of lactose, such as the sulfur-containing molecule IPTG

constitutive mutations

genes with these mutations produce enzymes regardless of lactose presence/absence

negative control of lac operon

in absence of lactose, lac operon is repressed, expression occurs when repressor fails to bind to the operator

allolactose

inducer, causes change in represor’s shape, preventing it from binding the operator

lacI- mutation

repressor is altered or absent so it does not bind the operator, leads to loss of regulation and continuous expression of the structural genes

lacOc mutation

the operator is altered and not recognized by a repressor, leads to loss of regulation and continuous expression of the structural genes

attenuation

stops transcription early if enough of an amino acid (like tryptophan) is already made

riboswitches

segments of mRNA that change shape when they bind small molecules, control whether transcription or translation continues

aptamer

ligand binding site on riboswitch

expression platform

downstream region of a riboswitch that translates the ligand-binding event in the upstream aptamer into a change in gene expression

sRNAs (small noncoding RNAs)

short RNAs that bind to mRNA to regulate translation (50-500 nucleotides), complementary to target mRNA

sRNA negative regulation

binds near ribosome binding site, blocks translation

sRNA positive regulation

prevents inhibitory structures from forming, helps ribosome bind

CRISPR-Cas

mechanism by which bacteria respond to specific bacteriophage attack by destroying invading phage DNA

contains repeats of identical DNA sequences and spacers (pieces of viral DNA from past infections)

CRISPR-Cas system steps

• Spacer Acquisition:

  • When phage DNA enters, Cas1 and Cas2 cut it and add fragments (spacers) into the CRISPR locus.

  • New spacers are added next to a leader sequence.

• crRNA Biogenesis:

  • CRISPR locus is transcribed → long RNA → processed into short crRNAs (each with one spacer).

• Target Interference:

  • crRNAs guide Cas nucleases to matching viral DNA during future infections.

  • Cas cuts the viral DNA → destroys it.