Synthesis of the amino acid arginine requires prior conversion of a precursor to ornithine, which is then converted to citrulline, which is then converted to arginine. Each of these three steps is under the control of a separate enzyme, coded for by a separate gene.
In wild type Neurospora, the synthetic pathway is intact, and spores are able to grow on minimal medium without added amino acids. In mutant Neurospora, some enzyme in the pathway is non-fuctional, so that spores are unable to grow on minimal medium. All of the mutants here are called arg- mutants, since in all cases supplementation of the minimal medium with arginine permits growth.
Among these mutants,
three classes are
distinguishable, depending on their growth response to the addition of
other amino acids in the pathway. Supplementation of the medium with
amino acid downstream from an enzymatic block permits growth,
the pathway is intact from that point on. For example, any of orn,
or arg permit growth of Class I mutants, since only the
enzymatic step is blocked. Supplementation with an amino acid upstream
from the block will not permit growth, since the spores are
unable to convert these precursors to arginine. For example, neither orn
nor cit permit growth of Class III mutants, in the
of the final enzyme in the pathway. Then, the observation of Class
mutants, in which the addition of cit permits growth and orn
does not, implies that cit occurs downstream and orn
upstream from the block, that is, that orn precedes cit
in the pathway.