Transactivation of a plasmid-borne bacteriophage T4 late gene

We examined how a plasmid-borne T4 late gene is activated by infecting T4 phage (transactivation). A gene fusion system was developed where expression of a late gene promoter fused to the lacZ gene may easily be followed by measuring beta-galactosidase activity. Considerable transactivation can occur, provided that the infecting phage contains a mutation which abolishes the denB-encoded endonuclease, and that the gene 46-encoded exonuclease is functional. The level of transactivation was correlated with the formation of high molecular weight DNA composed of tandem repeats of plasmid DNA. The formation of these molecules and subsequent transactivation depended on DNA replication and homology between phage and plasmid DNAs. Also the capacity of bacteriophage T4, grown on cells containing a plasmid-borne T4 gene, to transduce the plasmid provided indirect evidence of the formation of these tandem-repeat molecules. A good correlation was established between the ability to transduce and the presence of sequence homology between the phage and the plasmid. However, the requirement for phage/plasmid homology is no longer prerequisite if transcription from the plasmid is permitted by introducing an alc mutation into the infecting phage, presumably because this allows DNA replication to start through RNA priming.