Please explain your answer if you are not sure please dont a

***Please explain your answer; if you are not sure please don\'t answer. I don\'t want to post the same question many times just to get a correct answer. Thanks!***

20. Tryptophan is required by all living cells, as it is one of the component amino acids in most proteins. The compounds listed below are intermediates in the synthesis of tryptophan from the starting material chorismic acid. Extracts from eight tryptophan auxotrophs were tested for ability to synthesize tryptophan if supplied with the indicated compounds. Anthranilate Chorismate CDRP IGR A. Draw a pathway of tryptophan synthesis consistent with the responses shown trp synthesis no trp synthesis). B. Show the step in that pathway in which each mutant is blocked. C. Write two pairs of mutations that might fail to complement each other.

Solution

A. A probable pathway for Trp synthesis based on details mentioned is: Chorismate – Anthranilate – PRA – CDRP – IGP – Indole – Trp.

B. Mutant 4 is blocked at chorismate to anthranilate, mutant 7 blocked at anthranilate to PRA, mutants 2, 6 and 8 blocked at PRA to CDRP, mutant 5 at CDRP to IGP and mutant 1 at IGP to indole conversion step. Mutant 3 possibly can’t intake any of the supplied nutrients.

C. Mutant pairs 2 x 6 and pair 6 x 8 can’t complement each other.

The reasons for above answers are:

The less number of ‘plus’ sign indicate that the blocked step is much downstream of the Trp synthesis pathway. The rationale behind this is that if the blocked step is upstream in the pathway, it can still synthesize Trp using lower intermediates, but if the block is downstream it won’t be able to synthesize Trp from any. Thus, I came up with this pathway concluding from the increasing number of intermediates it can grow on and simultaneously going upstream the pathway. The blocked step in a mutant can be concluded from the fact that it can’t metabolize that nutrient for Trp synthesis. Because mutants 2,6 and 8 are blocked at same step, they can’t complement each other.

(Please get back to me if still anything remains unclear.)