1) Leucine, isoleucine, and valine are all hydrophobic amino acids that are synt
ID: 35371 • Letter: 1
Question
1) Leucine, isoleucine, and valine are all hydrophobic amino acids that are synthesized as the end products of a shared pathway. Neurospora cannot grow without all three. Several intermediates have been identified, but I won?t trouble you with their names-let?s just call them A, B, and C. Mutations n this pathway were identified, all of which affect a step in this pathway. From the table below, which of the enzymatic pathways must be the one leading to these tree amino acids. Table 1 Growth of Neurospora crassa in minimal medium with various supplements Figure 1 Alternative pathway for the synthesis of leucine and valine in NeurosporaExplanation / Answer
The production of the enzymes of isoleucine and valine synthesis by Neurospora in response to specific end product-derived signals depends upon the presence of an effective leu-3 regulatory product and its effector ?-isopropylmalate (?-IPM).
In leu-3+ strains, the productin of threonine deaminase is repressed as a function of available isoleucine, acetohydroxy acid synthetase as a function of valine, and the isomeroreductase and dihydroxy acid dehydratase as a function of isoleucine and leucine.
In the absence of an effective leu-3 regulatory product, ?-isopropylmalate, or both, the production of isoleucine and valine biosynthetic enzymes is fixed at or is near fully repressed levels even under conditions of severe end product limitation.
Thus, in addition to its involvement in the regulation of expression of the three structural genes of leucine synthesis, the leu-3 ?-IPM regulatory product is necessary for its full expression of at least four genes specifying the structure of the enzymes of isoleucine and valine synthesis.
It is suggested that the leu-3 ?-IPM regulatory element may facilitate transcription of the genetically dispersed cistrons either by imposing specificity on ribonucleic acid polymerase for structurally similar promoters adjacent to each of the cistrons or by
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