9. OC mutants are changes in the DNA sequence of the operator that impair the binding of the lac repressor. Because an operator controls only the genes on the same DNA strand, it is cis (on the same strand).
12. A gene is turned off or inactivated by the “modulator” (usually called a repressor) in negative control, and the repressor must be removed for transcription to take place. A gene is turned on by the “modulator” (usually called an activator) in positive control, and the activator must be added or converted into an active form for transcription to take place.
21. The S mutation is an alteration in lacI such that the repressor protein binds to the operator, regardless of whether inducer is present. In other words, it is a mutation that inactivates the allosteric site that binds to inducer but does not affect the ability of the repressor to bind to the operator site. The dominance of the S mutation is due to the binding of the mutant repressor, even under circumstances when normal repressor does not bind to DNA (i.e., in the presence of inducer). The constitutive reverse mutations that map to lacI are mutational events that inactivate the ability of this repressor to bind to the operator. The constitutive reverse mutations that map to the operator alter the operator DNA sequence such that it will not permit binding to any repressor molecules (wild-
24. Mutations in cI, cII, and cIII would all affect lysogeny: cI encodes the repressor, cII encodes an activator of PRE, and cIII encodes a protein that protects cII from degradation. Mutations in N (an antiterminator) also would affect lysogeny because its function is required for transcription of the cII and cIII genes, but it is also necessary for genes having roles in lysis. Mutations in the gene encoding the integrase (int) also would affect the ability of a mutant phage to lysogenize.