Establishment and Maintenance of Repression by Bacteriophage Lambda: The Role of the cI, cII, and cIII Proteins

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To define the events necessary for the establishment and maintenance of repression in a λ-infected cell, we have studied the requirements for efficient synthesis of the cI protein (“λ-repressor”). Three classes of λ mutants defective in the establishment of repression are also defective in the appearance of cI protein activity at the normal time. Two of these mutational classes (cII- and cIII-) probably result from inactivation of λ-specified proteins, but the third class (cy-) may involve a structural defect. We conclude that at least three regulatory elements are likely to be required for the normal turn-on of cI protein synthesis in an infected nonlysogenic cell: cII and cIII proteins and an “active” y-region of λ DNA. From these and other results, the complete role of cII and cIII proteins in the establishment of repression may involve a bifunctional regulatory activity: positive regulation of the cI gene and negative regulation of late genes. A possible molecular model for cII and cIII action is discussed. Since the cII and cIII genes are repressed by the cI protein under conditions of stable lysogeny, a separate mechanism is required for the maintenance of cI protein synthesis. After infection of a lysogen by cII- phage, the rate of increase of cI protein activity is substantially greater than after infection of a nonlysogen. From these and other results, the cI protein may also have a bifunctional regulatory activity: positive regulation of the cI gene and negative regulation of early lytic genes.

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