Question

In a diploid organism, the IND gene encodes a transcriptional activator that binds to an enhancer upstream of the EXP gene. The REP2 gene encodes a repressor protein that binds to IND and blocks its activation domain in the absence of an inducer called costar. In the presence of costar, REP2 protein no longer blocks the activation domain of IND. Both genes (IND and RE2) are expressed in all cells. There are complete loss of function mutants available for both IND (designated IND-) and REP (REP-).

How much EXP protein would you expect to see in the following conditions. Briefly justify each answer:

a). wild-type cells, no costar present

b). wild-type cells, costar present

c). homozygous REP2- mutant, costar absent

d). homozygous REP2- mutant, costar present e). homozygous IND- mutant, costar absent

f). homozygous IND- mutant, costar present

Answer 1

Note - A basal level of almost every gene is always maintained in a cell.

Basal level of expression means  that the protein is present in less than minimum required level thus unable to show desirable effect.

A.) Since no costar is present, REP 2 protein can easily bind to IND protein's activation domain thus preventing IND protein from binding to enhancer region of EXP gene and increase it's expression. so only basal level of EXP will be expressed in the cell.

B.) Since costar is present, then it will prevent REP2 from binding to IND and now IND can bind to enhancer of EXP, thus increasing the expression level of EXP gene. EXP expression will be more than just basal level .

C.) and D.) Since homozygous mutant of REP 2 will not have REP 2 gene thus no REP2 protein, hence costar's presence or absence will not make any difference because it's interacting REP2 protein will not be present , as a result IND protein can bind continuously or constitutively to EXP gene enhancer region thus unregulating it's expression and maintaining EXP expression on high level. Constitutive unregulation of EXP gene.

E.) and F.) Since IND gene is absent in mutants so regulation of REP2 gene by costar does not make any sense to IND regulation as REP 2 will not have IND interaction protein. So EXP gene will be expressed only on basal level.