Abstract. A new gene in bacteriophage X is described. The product of this gene cro prevents expression of immunity and regulates the expression of those genes to the left of the immunity region. cro-mutants have been isolated and characterized.Introduction. The Ci gene of bacteriophage lambda, which codes for the repressor,' is located between the two operons controlled by the repressor itself2-5 and is transcribed from the 1 strand of the DNA, as is the N operon.4 Occurrence of a mechanism controlling the expression of the CI repressor has been evidenced by the study of lysogens in which prophage induction does not affect the growth of the host: These carry derivatives of the N-defective thermoinducible (C1,57) prophage X, with an additional mutation blocking phage DNA

No abstract