Yeast selectable markers — Growth is specifically inhibited by α-aminoadipate when which marker gene is present (i.e., functional)?

Introduction / Context:
Genetic selections in yeast exploit toxic analogs that become lethal only in the presence of specific metabolic functions. α-Aminoadipate (α-AA) is a classic reagent used in LYS2-based selections to enrich for particular recombinants or counterselect for marker presence.

Given Data / Assumptions:

• LYS2 encodes an enzyme in the lysine biosynthetic pathway (α-aminoadipate pathway).
• α-Aminoadipate can be toxic to LYS2+ cells under selection conditions.
• URA3, HIS3, TRP1 are unrelated markers with different counterselections (e.g., 5-FOA for URA3).

Concept / Approach:
Counterselection relies on a toxic intermediate produced or processed when the corresponding gene is active. In α-AA selection, cells with functional LYS2 are sensitive; loss-of-function lys2 mutants are resistant and can grow, allowing targeted genetic manipulations.

Step-by-Step Solution:

Verification / Alternative check:
Protocols for lys2 selection routinely specify α-aminoadipate plates to enrich lys2 mutants.

Why Other Options Are Wrong:

• a/d/e: Not the cognate marker for α-AA toxicity.
• c: URA3 is counterselected with 5-FOA, not α-AA.

Common Pitfalls:
Confusing different counterselectable markers; memorize the specific analog: 5-FOA ↔ URA3; α-AA ↔ LYS2.

Final Answer:
LYS2