Control of enzyme amount: One/Two Component Regulators

Question 1

What is the purpose of one and two component regulators?

Answer 1

they regulate initiation of transcription to control enzyme amount, thus controlling flux through a pathway

Question 2

one component regulators are a

Answer 2

single protein that can detect a stimulus and mediate a response

they are usually DNA BINDING PROTEINS

they bind to DNA and affect transcription

Question 3

example of a one component regulator

Answer 3

the lac repressor in e coli

In the absence of lactose, the Lac repressor binds to the operator and keeps RNA polymerase from transcribing the lac genes. It would be energetically wasteful for E. coli if the lac genes were expressed when lactose was not present

allolactose is an inducer of lac operon: binds to the lac repressor and causes it to dissociate from the lac operator allowing transcription of the lac genes

Question 4

another example of a one component regulator

Answer 4

CAP (catabolite repression or activation) in e coli

when E. coli is placed on an agar plate containing only glucose and lactose, the bacteria will use glucose first and lactose second. When glucose is available in the environment, the synthesis of β-galactosidase is under repression due to the effect of catabolite repression caused by glucose

There is an inverse relationship between cAMP and glucose: the more glucose the less cAMP. CAP binds cAMP which activates it as a transcriptional activator, which will only happen when glucose levels are low.

Question 5

two component regulators

Answer 5

consist of a

histidine protein kinase

and a

response regulator

the histidine kinase detects the stimulus and autophosphorylates. It passes the phosphate onto an aspartate residue in the response regulator (phosphotransfer). RR’s are normally a transcription factors that become activated due to the conformational change that phosphorylation causes.

Question 6

example of a two component regulator

Answer 6

oxygen sensing in Sinorhizobium melitoti

FixL/FixJ ensures nitrogenase is only expressed in the absence of O2 (nitrogenase is irreversibly inhibited by O2)

FixL = histidine kinase
FixJ = response regulator (controls nitrogenase genes)

O2 present: FixL off = no FixJ-P = no nitrogenase
O2 absent: FixL on = FixJ-P = nitrogenase

FixL/J system regulates the nifA and fixK operons which control expression of nitrogenase

FixL senses O2 by binding HAEM