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 cell.  Alternatively, P4 may lysogenize the bacterial host and be maintained in an immune-integrated condition.  A key step in the choice between the lytic/plasmid vs. the lysogenic condition is the regulation of P4 alpha operon.  This operon may be transcribed from two promoters, PLE and PLL, and encodes both immunity (promoter proximal) and replication (promoter distal) functions.  PLE is a constitutive promoter and transcription of the downstream replication genes is regulated by transcription termination.  The trans-acting immunity factor that controls premature transcription termination is a short RNA encoded in the PLE proximal part of the operon.  Expression of the replication functions in the lytic/plasmid condition is achieved by activation of the PLL promoter.  Transcription from PLL is insensitive to the termination mechanism that acts on transcription starting from PLE.PLL is also negatively regulated by P4 orf88, the first gene downstream of PLL.  An additional control on P4 DNA replication is exerted by the P4 cnr gene product.