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

Echols, H., & Green, L. (1971). Establishment and Maintenance of Repression by Bacteriophage Lambda: The Role of the c I, c II, and c III Proteins. Proceedings of the National Academy of Sciences, 68(9), 2190–2194.