Regulation of the tryptophan operon of Escherichia coli integrated into the phage ϕ80 genome

Summary

Regulation of the expression of the tryptophan (trp) operon of Escherichia coli integrated into the genome of transducing phage ϕ80pts was studied by infecting sensitive or immune bacteria and measuring the activities of tryptophan biosynthetic enzymes.

In sensitive cells, expression of the trp operon in the genome of a transducing phage ϕ80pt190h is barely repressed by tryptophan and is mostly manifested by the “read through” transcription initiated at the phage promoter site, despite the fact that the phage carries the whole trp operon including the specific operator-promoter region. In ϕ80 lysogenic cells, however, transcription of the “early” genes is repressed by the phage immunity, and the trp operon is completely subject to the control by tryptophan. These results suggest that the repressor of the trp operon cannot prevent its transcription which was initiated at the preceding viral promoter on the same DNA. On the other hand, no appreciable read through transcription occurred with another transducing phage ϕ80pt66 (also carrying the trp operator-promoter region) upon infection of sensitive cells, and the trp operon was under normal control by tryptophan.

In relation to these observations, a “turn off” control of early messenger RNA synthesis similar to that found with phage λ was observed in the transcription of the ϕ80 genome.