Suppose that recombinant DNA technology was used to make a gene in which the structural genes of the lac operon were placed immediately downstream of the promoter and operator of the trp operon. This new operon is then introduced into the chromosome of an E. coli strain that does not have the normal lac operon. Therefore, the only way to get expression of β-galactosidase is from this artificial gene.
Bacteria with this artificial gene were grown under various conditions, and the level of β-galactosidase expression was measured. The effect of mutation of the trpR gene was also evaluated. The results from these experiments are given below:
presence of
tryptophan
presence of
glucose
presence of lactose
genotype
Level of β-galactosidase
expression:
present
absent
present
trpR+ trpO+
low
absent
present
present
trpR+ trpO+
high
present
absent
present
trpR- trpO+
high
absent
present
present
trpR- trpO+
high
Can someone please explain to me in detail why the levels of β-galactosidase happened to turn out like this?
Suppose that recombinant DNA technology was used to make a gene in which the structural genes of the lac operon were placed immediately downstream of the promoter and operator of the trp operon. This new operon is then introduced into the chromosome of an E. coli strain that does not have the normal lac operon. Therefore, the only way to get expression of β-galactosidase is from this artificial gene.
Bacteria with this artificial gene were grown under various conditions, and the level of β-galactosidase expression was measured. The effect of mutation of the trpR gene was also evaluated. The results from these experiments are given below:
presence of tryptophan | presence of glucose | presence of lactose | genotype | Level of β-galactosidase expression: |
present | absent | present | trpR+ trpO+ | low |
absent | present | present | trpR+ trpO+ | high |
present | absent | present | trpR- trpO+ | high |
absent | present | present | trpR- trpO+ | high |
Can someone please explain to me in detail why the levels of β-galactosidase happened to turn out like this?
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For this question, focus on four parts:
I, the gene that produces the repressor protein;
P, the promoter region where RNA polymerase binds to initiate transcription;
O, the Operator site, where the repressor binds. When the repressor binds, it prevents transcription; and
ZYA, the three structural genes, all of which are required for the successful utilization of lactose as a food source.
In the left column of the table below, we indicate the genetic composition at the lac operon of particular strains of E. coli cells. A "+" (plus sign) following the letter indicates the wild-type allele (or normally functioning version of the gene). For I, P, and ZYA, a "-" (negative sign) following the letter indicates the mutant allele (or non-functional version of the gene). For the O allele, a "C" indicates the mutant or constitutive allele. It is not functional in that the repressor cannot bind, but it is functional in that transcription can still occur.
Given the conditions stated in each column, please indicate for each genotype whether lac mRNA is produced and at what level by typing in one of the following words in each cell in the table:
"Abundant"
"Minimal"
"None"
The completely wild-type genotype has already been filled in.
Cellular conditions: | |||
Genotype | Glucose present (low cAMP) and no lactose | Glucose present (low cAMP) and lactose present | No glucose (high cAMP) and lactose present |
I+ P+ O+ ZYA+ | None | Minimal | Abundant |
I- P+ O+ ZYA+ | |||
I+ P- O+ ZYA+ | |||
I+ P+ Oc ZYA+ | |||
I+ P+ O+ ZYA- |