The following problems are to help you sharpen and deepen your understanding of the regulation of gene expression at the transcriptional level. These questions will be discussed in recitation on Nov. 14.

1. One can isolate rare mutations in the lac operator (lacO) gene that produce a non-inducible (super-repressed) phenotype. Like other lacO mutations, these mutations (called O⁰) are cis-dominant.

a) Propose a model for the molecular basis of O⁰ mutations.
b) Based upon your model, explain why O⁰ mutants are cis-dominant. (Consider the situation in O⁺/O⁰ and in O^c/O⁰.)
c) Based upon your model, make predictions about whether or not O⁰ mutations are epistatic to lacI^- (constitutive) mutations or to lacI⁺ (inducible).

2. The cAMP-CAP system provides positive control in the lac operon (superimposed upon negative control by the lacI gene) and in the ara operon (superimposed upon positive control by the araC gene). Based upon your understanding of these mechanisms, predict how each of the following mutations would affect expression and regulation of the lac and ara operons:

a) A mutation that inactivated the enzyme (adenylate cyclase) that synthesizes cAMP.
b) A mutation that inactivated the CAP protein.

3. Is there any protein in eukaryotes that has a function similar to that of the -factor in prokaryotes? If so, identify it and explain why its function is analogous to that of -factor.

4. Based upon your understanding of enhancer elements, would you expect mutations in an enhancer region to be cis-dominant or trans-dominant? Explain why.

5. Enhancers sometimes act in a tissue-or-cell-specific fashion. For example, if a muscle-specific enhancer is placed near a foreign gene and this is then reintroduced into a eukaryotic organism, one observes expression of the foreign gene only in muscle cells. Is the tissue-specificity of enhancers a sufficient explanation of tissue-specific gene expression? If so, why? If not, why not?