In the biosynthetic pathway for conversion from homoserine to methionine, you id

Question

In the biosynthetic pathway for conversion from homoserine to methionine, you identify a double mutant Met1/Met2. This mutant will grow only if which supplement(s) are added to the minimal media? (see picture)

(a) Experimental data Growth Medium Compound Mutant Minimal Minimal Minimal Minimal Minimal accumulating strain medium cysteine cystathionine homocysteine methionine in mutant Control prototroph None Homocysteine Met 1 Met 2 Cystathionine Met 3 Cysteine Met 4 Homoserine (b) order of intermediates in pathway Meet 4 Met 3 Met 1 Met 2 Homoserine Cysteine Cystathionine Homocysteine -t Methionine 02012 Pearson Education, Inc.

Explanation / Answer

The image shows that the mutant Met4 comprising of addition of all the 4 supplements in the minimal mdia follows up conversion of homoserine to methionine. It can be noted that rest of the Met-mutants comprise of only 1,2 or 3 of the supplements and can follow up the conversion of homoserine to either homocysteine, cystathionine or cysteine only respectively. Thus, complete conversion of homoserine to methionine is dependent upon present of all the four supplements provided in the minimal media.

Any system which is devoid of any of these four supplements will lead to halting the reaction at either of the steps. For example, a reaction mixture containing all nutrients except cystathionine will stop at step 3 and thus only cysteine will be obtained as a product.

These informations clealry suggest that minimal media supplemented with cystein plays the most crucial and final step of converstion of homoserine to methionine but all the four nutrients are absolutely required for this metabolic conversion.

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