There are several possible mutations in the trp operon:
1.
trpP - is a mutation in the promoter sequence that prevents RNA polymerase from binding to the promoter and initiate transcription of the trp operon genes.
trpOc is a mutation in the operator sequence that prevents the trp repressor protein from binding to the operator to block transcription of the trp operon genes.
trpR- is a mutation in the repressor protein that either prevents repressor protein from being made or produces a mutant repressor that cannot bind to the operator region.
trpRs is a mutation in the repressor protein that causes the repressor to be more sensitive to tryptophan levels. This mutant repressor binds the operator region even when tryptophan is low.
2. If you were to make a partial diploid with the genotype trp R+ trpOc / trpRs trpO+, what would happen? What is the dominance relationship between trpOc and trpRs?
a. trp genes will be expressed constitutively; trpRs is dominant to trpO c.
b. trp genes will not be expressed; trpO c is dominant to trpR s.
c. trp genes will not be expressed; trpR s is dominant to trpOc.
d. trp genes will be expressed constitutively; trpO c is dominant to trpRs.
e. trp genes will be expressed normally (repressible); trpO c and trpRs cancel each other out.
3.The trp repressor binds to the operator region of DNA as a dimer (two repressor protein subunits bound together as a unit).
You isolate a new mutant E. coli strain that you call Rnovel. In this strain, the repressor protein cannot attach to DNA (it cannot bind trpO+), but it can make dimers.
In this strain, how is the expression of the trp operon regulated?
a. constitutive
b. inducible
c. repressible
d. noninducible
4. You make a partial diploid of this new strain by introducing a wild type F' plasmid.
trpRnovel trpO+ / trpR+ trpO +
Given what you know about the "novel" mutation, how will the expression of the trp operon be regulated in this partial diploid?
a. Expression of trp genes will be constitutive. trpRnovel is recessive to trpR+.
b. The expression of trp genes will be repressible. trpRnovel is recessive to trpR+.
c. The expression of trp genes will be repressible. trpRnovel is dominant to trpR+.
d. The expression of trp genes will be constitutive. trpRnovel is dominant to trpR+.
5. Now predict the outcome of a partial diploid of your novel dominant-negative mutation (trpR d-) and a trpR s mutation.
trpRd- trpO+ / trpRs trpO+
How would trp genes be regulated in this partial diploid?
a. There will be no expression of trp genes. trpRd- is recessive to trpRs.
b. The expression of trp genes will be constitutive. trpRd- is dominant to trpRs.
c. The expression of trp genes will be constitutive. trpRd- is recessive to trpRs.
d. There will be no expression of trp genes. trpRd- is dominant to trpRs.