The pattern of binding of repressor and Cro to these sites determines whether the phage grows lytically or lysogenicaly. Launch the molecular model of the temperate bacteriophage repressor to explore its structure. Then, answer the questions. Launch molecular model Part A The pattern of binding of repressor and Cro to these sites determines whether the phage grows lytically or lysogenically. Which of the following accurately describes the regulation of lytic and lysogenic cycles by bacteriophage repressor? a. In the lysogenic cycle, Cro protein is produced, preventing the expression of the repressor protein; genes for viral replication are expressed.In the lytic cycle, repressor protein is produced, preventing the expression of Cro genes for viral replication are not expressed. b. In the lysogenic cycle, both repressor and Cro proteins are produced: genes for viral replication are expressed. In the lytic cycle, there is no expression of either repressor or Cro protein: genes for viral replication are not expressed c. In the lysogenic cycle, there is no expression of either repressor or Cro protein; genes for viral replication are not expressed. .In the lytic cycle, both repressor and Cro protein are produced; genes for viral replication are expressed. d. In the lysogenic cycle, repressor protein is produced, preventing the expression of Cro and genes for viral replication. In the lytic cycle, Cro protein is produced, preventing the expression of repressor protein

Each of these proteins binds with differing affinities and consequences to the three sites at OR (2). CI binds tightly to OR1 and weakly to OR2 and OR3; CI also binds cooperatively to OR1 and OR2. In the lysogenic state, CI bound to OR1 and/or OR2 represses expression of cro and lytic genes from PR, and CI bound to OR2 stimulates cI expression from PRM. CI also represses PL in the lysogenic state. Cro, by contrast, binds weakly to OR1 and OR2 and tightly to OR3. Cro is expressed from the PR promoter and, as its levels increase during the lytic cycle, it begins to bind to its operators. At relatively low concentrations, Cro binds to OR3; this represses PRM, although it is unclear whether this effect is important to normal lytic growth, because PRM is a weak promoter. At higher concentrations of Cro, such as those found late in infection, Cro also partially represses PR by binding to OR1 and/or OR2, so that early gene expression is reduced at later times (Fig. 1B; see also Fig. 5, which is published as supporting information on the PNAS web site). Cro also binds to three sites in the OL region, but this repression is probably only partial.