Question

Made up amino acids Bleucine and Isobleucine are synthesized in a series of steps from made up sugar Blorptose. These are essential for the organism to grow but can be supplied in the medium otherwise to complement the growth defect of mutants when amino acids cannot be synthesized. There are a number of chemical intermediates that are each synthesized by enzymes. Some enzymes in the pathway give both Bleu^- and Isobleu^- phenotypes, while others are only Bleu^- or Isobleu^-. You ask which mutants (labeled C,D,F,G,H,I,J) can be rescued by which compounds for growth on medium lacking both Bleucine and Isobleucine (pay attention to this- what does this mean?). These data and the phenotypes of each mutant are below.

Mutant	Phenotype	Blorpipic Acid rescue	Blorponol rescue	Screucine rescue	5m-screucine rescue	4m-screucine rescue	Bleucine rescue (control)	Isobleucine rescue (control)	Bleucine+Isobleucine (control)
C	Bleu-	No	No	No	Yes	No	Yes	No	Yes
D	Bleu-, Isobleu-	No	Yes	Yes	No	No	No	No	Yes
F	Bleu-	No	No	No	No	No	Yes	No	Yes
G	Bleu-, Isobleu-	Yes	Yes	Yes	No	No	No	No	Yes
H	Isobleu-	No	No	No	No	Yes	No	Yes	Yes
I	Isobleu-	No	No	No	No	No	No	Yes	Yes
J	Bleu-, Isobleu-	No	No	Yes	No	No	No	No	Yes

What is the most downstream enzyme in the production of Isobleucine? (Write in letter for the mutant that represents the enzyme)

Answer 1

Mutant J , G and D lacks both bluecine and isobleucine becuse it does not show activity in presence of individual bleucine and isobleucine.

Mutant H contain most downstream enzyme in the production of isobleucine.