yptophan synthase is one of the enzymes synthesized from the trp mRNA. In wild-t
ID: 261695 • Letter: Y
Question
yptophan synthase is one of the enzymes synthesized from the trp mRNA. In wild-type E. coli the trp mRNA nas a short half-life, but the tryptophan synthase half-life is much longer. To investigate how changes to the stability of the enzyme or its mRNA affect enzyme activity, two strains of E. coli were engineered. Strain A stabilizes the trp mRNA and strain B rapidly degrades tryptophan synthase. The wild-type, A, and B strains were grown in a medium with glucose as the sole carbon source. After several generations, tryptophan was added to all three cultures and tryptophan synthase activity was measured periodically. Select the statements that describe the expected change in tryptophan synthase activity after the addition of tryptophan Note: Evaluate each condition as a simple model, where changes in the stability of trp mRNA or tryptophan synthase do not elicit secondary effects in the cells In strain A, the trp mRNA is stable and tryptophan synthase will continue to be synthesized However, as the cultures grow the tryptophan synthase will be diluted and the amount of enzyme activity will slowly decrease. In the wild-type strain, the trp mRNA is degraded rapidly. However, since the tryptophan synthase is stable, the enzyme activity per cell will remain unchanged as the cells divide In strain B, since both the trp mRNA and tryptophan synthase are rapidly degraded, the enzyme activity will drastically decrease In strain A, the trp mRNA is stable but the presence of tryptophan in the medium will inhibit synthesis of tryptophan synthase and abruptly decrease enzyme activity In the wild-type strain, trp mRNA is unstable and will be degraded rapidly. As the cells continue to divide, tryptophan synthase will be diluted out and enzyme activity per cell will decreaseExplanation / Answer
In strain A, point#4 is expected, with a functional Trp Operon in E.coli. Therefore, the stable mRNA will allow formation of tryptophan, the last two steps of which is catalyzed by the enzyme tryptophan synthase. However, increased tryptophan may lead to feedback or attenuation, which will have the enzyme to stop fuctioning and thus decreasing its activity.
In strain B, since both mRNA and the enzyme are not stable and the amount of tryptophan made will be little, thus keeping the iperon functional to have more tryptophan produced, ideally, the enzyme activity should not be lowered, even if not increased. It is not true that the activity will be reduced. However, in presence of excess of tryptophan the enzyme activity will be highly reduced, as will be the case after few generations when the strain B is subjected to lorge amounts of trp.
In the wild type strain, since strains are grown in glucose contaning medium, the enzyme will be present in large amounts as glucose has stimulatory effect on tryptophan synthetase (established long back by freundlich in 1962). But it was also found that no free tryptophan is ever seen in a medium that otherwise contains glucose. Given these information, if we have tryptophan added excessively into the system, atleast for initial few generations, the enzyme activity should remain unchanged, as in point #2.
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