Inference of a haploid biosynthetic pathway
X, Y and Z
are metabolites in a biochemical
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-
mutant strains of Neurospora
that cannot grow on minimal medium:
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
arg-1 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]:
Homework: Are arg4, arg2, and arg1 different alleles of the same gene, arg? Explain why or why not?