Inference of a haploid biosynthetic pathway
X, Y and Z
are metabolites in a
A, B, & C are enzymes that convert a precursor metabolite to X, X to Y, and Y to Z, respectively.
A fully functional pathway permits growth.
Mutant strains are unable to grow:
Some enzymatic block prevents synthesis of some metabolite from its precursor
Growth occurs if any downstream metabolite is added (the mutant is "cured").
If the addition of a metabolite enables growth of a mutant strain,
then the block occurs before synthesis of that metabolite,
or, the metabolite occurs in the pathway after the block.
Consider seven arg-
strains of Neurospora
cannot grow on
some of them can grow when supplied with the amino acids arginine, citrulline, and/or ornithine
mutant strain grows
with addition of any of arg, cit, or orn:
block must precede all three,
=> block affects metabolism of precursor substance
mutant strain has a specific
requirement for arginine:
block must occur immediately before arg, after both orn & cit
=> arg is last product in pathway
arg-2 mutant strain is 'cured'
by citrulline, and not by ornithine:
block must occur before cit, and after orn
block prevents conversion of orn to cit
=> cit occurs after orn
Arginine biosynthesis as inferred from preceding data [an alternative presentation]: