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emeraldjellyfish291Lv1

28 Sep 2019

The lac operon is the classic "Inducible system" for gene control, while the trp operon is the classic "repressible system". A key difference between these systems involves the manner in which the repressor proteins control transcription. Compare and contrast the LacI repressor protien with the TrpR repressor protein

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Hubert KochLv2

28 Sep 2019

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1) In the wildtype (the DNA sequence most commonly found in nature) lac operon, transcription is severely down-regulated. A mutation in lacI that causes an early stop codon in the sequence (called a lacI - strain) results in an operon that has high levels of transcription of the polycistronic m-RNA. What type of regulatory protein is the LacI protein (hint: we covered this type of protein in lecture)?

The Lac regulatory protein is called a repressor. (is this correct)

2) If a bacterium with a wildtype lac operon is grown in the presence of lactose, high levels of transcription result for a while, however, eventually transcription is again down-regulated. If, on the other hand, the sugar IPTG is present (instead of lactose), transcription levels are high and remain high. What function do IPTG and lactose share? How are they differently affected by the operon product? (need help with this question)

Lactose and IPTG affected by the operon are both inducers of the Lac-Operon. (is this correct)

3) What is âepigeneticsâ? What forms of gene regulation are usually involved in epigenetic control of a trait? Why do we have such a limited understanding of epigenetic processes? (need help with this question)

Epigenetics is the study of phenotype changes that do not change the DNA sequence. Genes can be turned on and off based on environment situations of past generations, (parents or even grandparents) were exposed. Examples of some environmental factors could be diet, stress, smoking, drinking alcohol, and diseases. Histone modification and methylation are usually involved in epigenetic control of a trait. (is this correct)

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 Î²-galactosidase is from this artificial gene.

Bacteria with this artificial gene were grown under various conditions, and the level of Î²-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 Î²-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 Î²-galactosidase happened to turn out like this?

ochremarten246

a) If a lacI - bacterium is grown in the presence of the sugar X-gal, a blue dye appears; however, the concentration of X-gal decreases over time. What has happened to the X-gal?

b) If a wildtype bacterium is grown in the presence of the sugar X-gal, transcription of the polycistronic m-RNA does not occur. What is X-gal unable to do? Without adding lactose (or genetically modifying the bacteria), how could you fix this problem (can you fix this problem by adding IPTG)

Can you explain the following answer in simple terms? Still do not understand what happens to the X-gal, and what x-gal is unable to do. Thank you

Answer A)

X-gal stands for 5-bromo-4-chloro-3-indolyl-Î²-D-galactopyranoside which has glycosidic bond between galactose and 5-bromo-4-chloro-3-hydroxyindole. It gets cleaved by beta-galactosidase translated from lac operon.

If lacI- mutant is provided with X-gal, because of mutant lacI gene, no repressor will be formed and hence the operon is swiched on.

The enzyme beta-galactosidase cleaves X-gal which results in decreased concentration of X-gal with time.

X-gal gets cleaved to form 5-bromo-4-chloro-3-hydroxyindole which then dimerizes to derivative of indigo and hence blue color appears.

Answer b

X-gal cannot form allolactose or its analogue (inducer) and so in wild type where lacI gene is normal and forms repressor. So, in absence of allolactose, repressor inducer complex is not formed and so, operon remains switched off and hence no beta-galactosidase is formed.

This problem can be solved by adding IPTG i.e. Isopropyl beta-1-thiogalactopyranoside which mimics the action of allolactose and so can switch on the operon.

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