On the Mechanism of Unrepressed Galactosidase Synthesis Controlled by a Transducing Phage

Abstract

Phage PI dl, a defective transducing phage derived from PI, carries a lac operon derived from a previous host (Luria et al., 1960). With PI dl phage carrying i−z+ alleles of the repressor and the β-D-galactosidase genes, peculiar regulatory effects are observed (Revel et al., 1961a, b, c). Notable among these are the constitutive synthesis of galactosidase following infection of i+z− cells (escape synthesis) and a similar constitutive synthesis following ultraviolet induction of bacteria of the genetic constitution i+z−(Pl dl i−z+) (derepressed synthesis). Constitutive enzyme production, at lower levels, is observed also with PI dl i+z+ phage in analogous situations. When these findings were first reported, two interpretations were considered: (a) a topological effect, by which a repressor would not be fully effective on the operator site when this was part of a vegetative phage element; or (b) a multiplicity effect, that is, a failure of repression when the ratio...