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To define the events necessary for the establishment and maintenance of repression in a lambda-infected cell, we have studied the requirements for efficient synthesis of the cI protein ("lambda-repressor"). Three classes of lambda 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 lambda-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 lambda 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.

Establishment and Maintenance of Repression by Bacteriophage Lambda: The Role of the c I, c II, and c III Proteins