J. Virol., 06 1997, 4502-4508, Vol 71, No. 6
Copyright © 1997, American Society for Microbiology

Derepression of prophage P2 by satellite phage P4: cloning of the P4 epsilon gene and identification of its product

T Liu, SK Renberg and E Haggard-Ljungquist
Department of Genetics, Stockholm University, Sweden.

Escherichia coli phage P4 lacks all of the genetic information necessary for capsid, tail, and lysis functions. P4 is therefore dependent on a helper phage, such as P2, for lytic propagation. During P4 superinfection of a P2 lysogen, the P2 prophage is derepressed by the action of the P4-encoded epsilon gene. We have cloned the epsilon gene and identified the 10-kDa E protein. The epsilon gene product is the only P4 protein required to derepress prophage P2, which leads to in situ P2 DNA replication. A two-plasmid derepression assay system has been developed to examine the derepression activity of E. The reporter plasmid contains the two face-to-face promoters, Pe and Pc, involved in the lysis-lysogeny transcriptional switch of phage P2 and the immunity repressor C. The Pe promoter is coupled to a cat reporter gene. In the construct, the C repressor is transcribed from the Pc promoter and represses the Pe promoter, which mimics the in situ-repressed P2 prophage. The E protein is supplied in trans from a compatible plasmid in which the epsilon gene is under the control of the T7 promoter. We show here that in the two-plasmid assay system, induction of the E protein derepresses the Pe promoter. The ash9 mutation, which is located upstream of the epsilon gene, enhances the E-mediated derepression of the Pe promoter. The purified E protein shows no specific DNA binding activity, and the implications of this are discussed.

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