Practice question

In pathway A→B→C→D where enzymes named for products knockout of early enzyme B prevents synthesis of B and consequently substrate for downstream steps C and D. Overexpressing enzyme C converting C to D cannot rescue because substrate C never produced; pathway remains blocked upstream despite increased catalytic potential downstream. No endogenous downstream intermediates exist to drive flux so cell auxotrophic. Only complete medium supplying final product D plus all other growth factors bypasses entire endogenous biosynthesis restoring growth. This illustrates epistasis of early block over later overexpression dosage effect.