STUDIES OF HEAT-INDUCIBLE h PHAGE. 111. MUTATIONS IN CISTRON N AFJ3ECTING HEAT INDUCTION1

INDUCTION of a lysogenic bacterium can be defined as an event that results in a change in the host phenotype attributable to the presence of a prophage. The consequences of induction of K-12 ( A ) include loss of immunity to A, cell death, and synthesis of phage-specified enzymes. Increased synthesis of h mRNA occurs shortly after exposure of a lysogen to an inducing agent (ATTARDI et at. 1963; SLY, ECHOLS and ADLER 1964) and has been observed in lysogens in which cell killing and phage replication are blocked by chloramphenicol (GREEN 1966). Studies of h mutants suggested that the protein product of gene CI prevents X induction (JACOB and CAMPBELL 1959; JACOB, SUSSMAN and MONOD 1962). It has been proposed that the CI product represses the synthesis of mRNA for one or more enzymes required for h replication (JACOB and MONOD 1961). Mutations that result in induction when X lysogens are heated above 35°C map in the CI gene of X and are called CIt. A correlation between the locations of the t mutant sites on the Cr map and the properties of the corresponding lysogens led me to propose a model for the repression of h ( LIEB 1965,1966a). It was suggested that the C, product acts as a repressor when it is combined with a nascent protein, which is the product of an operator gene. The nascent protein was assumed to be an enzyme whose activity was inhibited by the Cr product. Mutations in the gene for this protein might be expected to influence its interaction with the C, product, and thus affect the heat-inducibility of prophages with mutations in CI. In an attempt to locate the gene for this hypothetical protein, lysogens containing mutations in CI and also in genes N , 0, P and R were prepared. The effects on heat induction of mutations in genes 0, P, and R are clearly distinguishable from those of N mutations, and will be reported elsewhere. This study concerns heat induction of X prophages with mutations in both Cr and N . Mutations in gene N result in a deficiency in the amount of a specific exonuclease which appears in X lysogens after the application of inducing agents (KORN and WEISSBACH 1963; RADDING 1964a, b) . As shown below, these mutations alter the ability of prophages containing CIt to be induced at high temperatures.