lectures 8 & 9 - control of gene expression in bacteria Flashcards
whats the def of a gene
a DNA segment that contributes to phenotype/function. In the absence of
demonstrated function a gene may be characterized by sequence, transcription,
or homology
what is an operon
a unit of bacterial gene expression and regulation, including
structural genes and control elements in DNA recognized by regulator gene
product(s)
how long are mRNA ½ lives
short: 1-3 min
what is a promoter
where RNA polymerase binds
what is an operator
where regulatory proteins bind and this controls what happens at the promoter
where are the promoters/operators in regards to structural genes and each other
upstream but can be in either order
what is constitutive gene expression
genes expressed most/all time
what is inducible gene expression
set of genes that are transcribed only as response to something in the environment
what is repressible gene expression
gene expression switched off in response to substance in env (often anabolic or biosynthetic pathways)
what type of expression is the lac operon
inducible system
when are lacZ,Y,A expressed and what prevents this when it is not there
only expressed when lactose (or analogues) are present
when no lactose, the lacI repressor binds to DNA at operator preventing RNA polymerase binding at promoter (there is a very low level of lac expressed)
when there is lactose, LacI has conformational change
how do operon inducers usually compare to substrates for the enzymes
usually small molecules that are identical or very close to substrate for the enzymes
what are trans-acting mutations
mutations that can act at a distance, not simply in the DNA
molecule in which they occur. (Usually affects production of a diffusible protein)
what are cis-acting mutations
site affects the activity only of sequences on its own molecule of
DNA (or RNA) (if affects several genes, implies that the site does not code for
protein, but is in a regulatory region)
is the lac mRNA stable and what does this mean
no its very unstable so induction can be rapidly reversed
how many binding sites does the lac repressor have and what are they for
2, 1 for operator and one for inducer
what type of mutations are mutations in lacI gene and why
trans-acting bc they affect expression of all lacZYA
what type of mutations are mutations in operator gene and why
cis-acting bc they only affect genes on the contiguous stretch of DNA
what is the inducer for the lac repressor
allolactose (changes the shape)
what does it mean to be negatively regulated
in absence of induced, expression is blocked (neg controlled) by repressor
what is glucose’s effect on the lac operon and why
regulates it positively becuase if you have glucose it’s more favorable to use glucose than lactose
how do concentrations of glucose and cAMP relate and why
inversely related due to direct competition between glucose transporter and adenine cyclase for the cofactor
how does cAMP affect CRP/lac operon
CAP or CRp is a positively regulator protein activated by cAMP. often needed for RNA pol to initiate transcription at operon. cAMP binds to CRP and activates it. the protein binds to DNA and increases operon transcription efficiency
what type of operon is the tryptophan operon
repressible
