Question

refer to the table to answer the following question. which step is catalyzed by the enzyme encoded by the met 4 gene? methionine → homoserine homocysteine → methionine cystathionine → homocysteine cysteine → cystathionine homoserine → cysteine met 1 met 2 met 3 met 4 prototroph control mutant strain minimal medium minimal + cysteine minimal + cystathionine minimal + homocysteine minimal + methionine homoserine → cysteine cysteine → cystathionine cystathionine → homocysteine homocysteine → methionine none homoserine cysteine cystathionine homocysteine compound accumulating in mutant

Explanation:

Step1: Analyze Met 4 gene function

The Met 4 gene - encoded enzyme catalyzes a step in the methionine - synthesis pathway. We look at the growth of mutants on different media and the accumulation of compounds.

Step2: Check growth requirements

The mutant strain with Met 4 mutation cannot grow on minimal medium but can grow on minimal medium supplemented with homocysteine or methionine. This indicates that the step before homocysteine in the pathway is affected.

Step3: Identify the step

From the pathway and the growth data, the Met 4 gene - encoded enzyme catalyzes the step from cystathionine to homocysteine.

Answer:

cystathionine → homocysteine