MULTIPLE REGULATORY MECHANISMS CONTROLLING PHAGE-PLASMID P4 PROPAGATION

Bacteriophage P4 autonomous replication may result in the lytic cycle or in plasmid maintenance, depending, respectively, on the presence or absence of the helper phage P2 genome in the Escherichia coli host ce ll. Alternatively, P4 may lysogenize the bacterial host and be maintai ned in an immune-integrated condition. A key step in the choice betwee n the lytic/plasmid vs. the lysogenic condition is the regulation of P 4 alpha operon. This operon may be transcribed from two promoters, P-L E and P-LL, and encodes both immunity (promoter proximal) and replicat ion (promoter distal) functions. P-LE is a constitutive promoter and t ranscription of the downstream replication genes is regulated by trans cription termination. The trans-acting immunity factor that controls p remature transcription termination is a short RNA encoded in the P-LE proximal part of the operon. Expression of the replication functions i n the lytic/plasmid condition is achieved by activation of the P-LL pr omoter. Transcription from P-LL is insensitive to the termination mech anism that acts an transcription starting from P-LE. P-LL is also nega tively regulated by P4 orf88, the first gene downstream of P-LL. An ad ditional control on P4 DNA replication is exerted by the P4 cnr gene p roduct.