Temporal mRNA regulation in phage systems

Roughly halfway through the life cycle of the phage this promoter enters the cell, transcription occurs by T7 RNA polymerase, and the structural proteins and lysis enzymes are synthesized. Lysis does not occur until phage particles have been assembled. 

Phage T4 has numerous promoters only a few of which can be recognized by E. coli RNA polymerase. However, unlike T7, the late promoters are made available by successive modification of the E. coli enzyme. These modifications are of two types addition of phage-encoded protein subunits and chemical modification of preexisting subunits. Temporal regulation occurs because the gene responsible for the first modification is encoded in the first set of mRNAs, that for the second modification in the second set, and so on. To ensure that the late mRNA, which encodes the structural proteins and the lysis enzyme, is not synthesized until adequate DNA has been made by the replication system, the template for this late mRNA cannot be parental T4 DNA but must be a replica. 

Genetic map of the regulatory genes of phage k. Genes are listed above the line sites are below the line. The mRNA molecules are heavy lines. The dashed black arrows with thin lines indicate the sites of action of the N, Cro, and Q proteins. 

---