Interactions between a satellite bacteriophage and its helper

P4 is a satellite phage which relies on a helper such as P2 to supply the gene products necessary for particle construction and cell lysis (Six, 1975). P4 can activate the expression of late genes from a P2 helper phage, using a mechanism different from that employed by the helper. In the presence of P4, replication of P2 DNA is not required for late gene expression (Six & Lindqvist, 1971), and the polar effects of P2 amber mutations are suppressed.Despite its small size P4 codes for two late proteins as well as two early proteins. One of the P4 early proteins is that the product of gene α. The expression of P4 late genes is stimulated by the helper phage. Thus the P2 and P4 chromosomes exhibit reciprocal transactivation.The presence of the P4 genome causes the P2 head proteins to form a head smaller than that found after infection by P2 (Gibbs et al., 1973). P4 late proteins associate with head-like structures and may determine the small size of P4 heads.