Mutants in the y region of bacteriophage X constitutive for represser synthesis: Their isolation and the characterization of the Hyp phenotype

Bacteriophage λhyp mutants have been isolated as survivors of Escherichia coli K-12 bacteria lysogenic for λNam7am53cI857. The hyp mutants are characterized by (i) their localization in the y region very close to the immλ/imm 434 boundary, (ii) polarity on O gene expression, (iii) immediate recovery of X immunity at 30°C after prolonged growth of λ Nfam7am53cI857 hyp lysogens at 42°C even in the presence of an active cro gene product, (iv) ability of phage λv2v3vs326 but not λv1v2v3to propagate on glcI +hyp lysogens, (v) inability to express X exonuclease activity after prophage induction, and (vi) inviability at any temperature of phage carrying the hyp mutation. All these properties are referred to collectively as the Hyp phenotype. We show that the Hyp phenotype is due to ell-independent constitutive cl-gene-product synthesis originating in the y region, which results in the synthesis of anti-cro RNA species, and constitutive levels of cro gene product present even in λcI +hyp lysogens. A model is presented which is consistent with all the experimental observations.