#Question id: 16127

You are studying regulation of the yeast enzyme glutamine synthetase (GS), which is encoded by the GLN1 gene. You have isolated two mutants, designated  gln2– and gln3–, that give decreased GS activity. Mating of either gln2– or gln3– haploids to wild type produces heterozygous diploids that show normal amounts of GS expression. When you cross either a gln2– or gln3– haploid to a gln1– strain the resulting diploids show normal expression of GS.

The GLN1 gene shows a rather complex regulation in response to different amino acids. When either glutamate (glu) or glutamine (gln) is added to the medium, the amount of GS expression diminished and when both glutamate and glutamine are added to the medium GS expression is shut off completely. The effects of different mutants on the response to glu and gln are shown below.

Which of the models from best fits these experimental results? Complete the model for the regulation of GLN1 that includes the effects of glu and gln.