Purification and characterization of the bacteriophage P4 delta protein

The bacteriophage P4 delta protein is a transcriptional activator of the late genes of P4 as well as the late genes of its helpers, such as bacteriophage P2. delta was purified, using a variation of the MalE fusion system. With this method we purified two forms of delta: a fusion of MalE and delta and a unfused form. The fusion by itself is not active in vivo or in vitro, but the mixture of the fusion and the unfused delta is active in both. Using nitrocellulose filtration and gel mobility shift assays, we show that delta binds DNA, and using DNase I footprinting, we show that delta binds to sequences centered at approximately -55 in the two late promoters of P4 as well as the four late promoters of its helper P2. In addition, the P4 sid promoter contains a second delta binding site centered at -18.