Regulation of genes in prokaryotes

Beedle and Tatum experiment

proved one gene, one enzyme theory, mutated conidia so they would fail to grow on minimal media, tested with different supplemented medias, addition of arginine restored growth

gene

unit that controls the inheritance of one character or one attribute of phenotype; a unit that is not subdivisible by recombombination, smalles unit that is capable of independent mutation

operon

cluster of structural genes and regulatory regions, transcribed as polycistronic region

structural gene

encodes an enzyme or a protein

promotor

binds RNA polymerase

operator

binds regulatory proteins

constitutive gene expression

gene expressed most/all of the time; typical of basic metabolic processes

inducible gene expression

gene is expressed in response to a substance’s presence in the environment; typical of catabolic enzymes

repressible gene expression

gene expression is switched off in response to a substance’s presence in the environment; typical of anabolic pathways

lac operon

an inducible system with 3 structural genes (lacz, lacY, lacA), induced by allolactose (a derivative of lactose), in absence of lactose a repressor (lacl) binds to DNA at operator preventing DNA from binding to promotor, lactose causes lacl to undergo conformational change and unbind from operator

lacz

structural gene for B-galactoside, which cleaves lactose

lacY

structural gene for permease which transports lactose into a cell

lacA

structural gene for transacetylase, which acetylates galactosides, a toxic metabolite of the process

trans-acting mutations

mutations that can act at a distance, not simply in the DNA molecule in which they occur

Cis-acting mutations

mutation affects the activity of sequence only on its own molecule of DNA or RNA

Hfr Strain

a bacterial strain called high frequency recombination. can transfer its genetic material to a second bacterial cell.

PaJaMo experiment

identified trans-acting mutant that always produced B-galactoside even in the absence of an inducer, suggests mutation in lacl

identified cis acting mutant that could not generate B-galactoside but can produce permease, suggests mutation in lacz

combined l+z+ donor with l-z- recipients by mating, z+ enters recipients and makes B-galactoside even in presence of enhancer until l+ also enters donor cell

O^c mutants

cis acting mutations that have a mutation at the binding site of the repressor, meaning it cannot bind a repressor and the structural genes are always expressed even in absence of an inducer

gene+

indicates the wildtype non-mutated gene

gene-

indicates the mutated gene

negative regulation

in absence of a molecule, expression is blocked by a repressor - can be induced by this molecule

positive control of lac operon

regulated by glucose concentration, when there is glucose and lactose, there is not as much B-galactoside produced as if there was only lactose. Glucose prevents transcription of the operon , low glucose allows cAMP to bind to cAMP receptor protein and increase efficiency of transcription, high glucose levels result in lower cAMP levels and less efficient transcription

catbolite activator protein (CAP)

another name for the cAMP receptor protein (CRP)

effect of CAP-cAMP dimer

binds 30 bp DNA, introduces a bend which may be a more efficient substrate for transcription

Repressible Tryptophan operon

Tryptophan absent, repressor mRNA transcribed, inactive repressor, enzymes of tryptophan synthesis pathway transcribed and translated

tryptophan present, repressor mRNA transcribed, ACTIVE repressor, enzymes of tryptophan synthesis pathway NOT produced

Attenuation

premature termination of transcription

attenuation in tryptophan operon

leader mRNA contains 2 tandem trp codons which are generated when trp levels are high, these pair to other parts of mRNA to form structure A when tryptophan is high causing ribosome to fall off at stop terminator after site two and terminator hairpin between sites 3 and 4; forms structure B when in tryptophan starvation, where ribosome can attach at site one and continue all the way to site 4

His

a attenuated operon, responsible for histidine biosynthesis, 7 His residues in leader peptide

Ilv

a attenuated operon, responsible for isoleucine, leucine, and valine biosynthesis, 5 Ile, 3 leu and 6 Val in 32-residue leader peptide

riboswitches

part of an mRNA molecule that can directly bind a small target molecule where binding of the target affects gene expression; more than 20 types in bacteria each binding a different ligand

riboswitches control of transcription

if no ligand is present, transcription occurs

if the ligand is present it induces conformational change and no transcription occurs

riboswitches control of translation

interfere with initiation, prevent ribosome binding at shine-dalagarno site