Biosynthetic Pathways and Auxotrophic Mutants

What is a biosynthetic pathway

A sequence of enzyme-catalyzed reactions that converts a starting precursor into a final product.

What is an intermediate in a biosynthetic pathway

A substance produced in one reaction and used as the substrate for a later reaction.

What is the end product of a biosynthetic pathway

The final substance produced by the sequence of reactions.

What is a precursor

The starting substance from which a pathway produces intermediates and an end product.

What causes a mutant to be blocked in a biosynthetic pathway

A mutation that prevents a required enzyme from functioning properly.

What does a plus sign (+) mean in a growth table

The strain grows when provided with the tested substance or medium.

What does a minus sign (−) mean in a growth table

The strain does not grow under the tested condition.

How can adding a biosynthetic intermediate help an auxotrophic mutant grow

The added intermediate can bypass the reaction that the mutant cannot perform.

What does growth after adding an intermediate tell you about a mutant

The intermediate is downstream of, or bypasses, the mutant’s blocked reaction.

What does failure to grow after adding an intermediate suggest

The supplied substance does not bypass the defective step under the tested conditions.

How do you determine the order of intermediates from mutant growth data

Compare which intermediates rescue each mutant; intermediates that rescue a mutant must be able to bypass its block.

How do you determine where a mutant is blocked in a pathway

Identify the earliest intermediate that restores growth and infer that the defective reaction occurs before that intermediate.

If a mutant grows only when given the final product, what does that suggest

Its block occurs at a step that cannot be bypassed by the tested earlier intermediates.

If a mutant grows when given intermediate C or the final product, but not intermediate B, what does that suggest

The block is between B and C, assuming the pathway proceeds from B to C.

If a mutant grows when given intermediates C, D, and the final product, but not A or B, what can you infer

Its defective step is upstream of C, although the exact location depends on the established pathway order.

Why can different mutants require different supplements

Their mutations may disrupt different enzymes at different steps of the same pathway.

Why is a wild-type strain useful when interpreting a growth table

It provides a control for whether the tested media and supplements support normal growth.

What does it mean if several mutants have the same growth pattern

They may be blocked at the same pathway step, but additional tests may be needed to determine whether their mutations affect the same gene.