#Question id: 16128

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. 

Next, you decide to evaluate the promoter for the GLN1 gene. To do this you first fuse the promoter region to the LacZ coding sequence and then place this hybrid gene on an appropriate yeast plasmid. You find that cells carrying the hybrid gene express activity under the same conditions that GS is expressed in wild type cells, meaning that the promoter region you have selected contains all of the necessary cis-acting sequences for normal regulation. The figure below shows the effect of different 50 bp deletions in the promoter region on the amount of ß-galactosidase activity expressed by the reporter gene. how would you expect a gln2– gln3– double mutant to behave?