#Question id: 2860
#Unit 2. Cellular Organization
Lactose metabolism in E. coli is carried out by two enzymes, with possible involvement by a third. The genes for all three enzymes are clustered together and transcribed together from one promoter, yielding a polycistronic message. These three genes, linked in function, are therefore also linked in expression. They are turned off and on together.
|
Negative control |
keeps the lac operon repressed in the absence of lactose. |
|
positive control |
keeps the operon relatively inactive in the presence of glucose, even when lactose is present. |
|
when all the glucose is gone and lactose is present, a mechanism should exist. |
|
|
is mediated by a factor called catabolite activator protein (CAP), which, in conjunction with cyclic-AMP, stimulates transcription. |
#Question id: 2861
#Unit 2. Cellular Organization
What is the result of regulatory mutation in given lac operon in merodiploids
#Question id: 2862
#Unit 2. Cellular Organization
What is the result of regulatory mutation in given lac operon in merodiploids,
#Question id: 2863
#Unit 2. Cellular Organization
The promoter of the araBAD operon from E. coli is activated in the presence of arabinose and the absence of glucose and directs expression of genes encoding enzymes required for arabinose metabolism. Unlike the cases of lac and gal genes, where a repressor and an activator work together, here two activators work together: AraC and CAP
|
+ arabinose |
AraC binds that sugar and adopts a configuration that allows it to bind DNA as a dimer to the adjacent half-sites, araI1 and araI2 |
|
– arabinose |
the AraC dimer adopts a different conformation and binds to araO2 and araI1, there is no monomer at site araI2, and so the protein cannot activate the araBAD promoter (araPBAD). This promoter is also controlled by CAP. |
#Question id: 2864
#Unit 2. Cellular Organization
In E. coli, the five contiguoustrp genes encode enzymes that sythesize the amino acid tryptophan. These genes are expressed efficiently only when tryptophan is limiting. The genes are controlled by a repressor, just as the lac genes are, although in this case, it is the absence of its ligand (tryptophan) that relieves repression, match trp genes to their gene product,
|
Trp genes |
Gene products |
|
trpE |
anthranilate
synthetase |
|
trpD |
phosphoribosyl
anthranilate transferase |
|
trpC |
phosphoribosyl
anthranilate isomerase-indole glycerol phosphate synthetase |
|
trpB |
tryptophan
synthetase beta |
|
trpA |
tryptophan
synthetase alpha |
#Question id: 2865
#Unit 2. Cellular Organization
The trp operon is controlled by repression and attenuation, providing a two-stage response to progressively more stringent tryptophan starvation. But attenuation alone can provide robust regulation: other amino acid operons such as leu and his rely entirely on attenuation for their control. Correct number of codons in leader peptide of leu & his operon in a row;