Evolutionary relationship between plasmids and phages: phasmid P4 as a model.

Phasmid (phage-plasmid) P4 is capable of propagating as a temperate phage, lytic helper-dependent phage, and as a plasmid (pP4vir1). Alteration of cellular growth conditions can be used to manipulate the copy number (n) of pP4vir1 from low (n = 1-2) to high (n = 30-40). The levels of gene expression of P4 in the plasmid and phage states are radically different. While there is high expression of P4 proteins in the phage state, low levels of gene expression are found in the plasmid mode of propagation thereby allowing pP4vir1 to remain intracellularly without perturbing host homeostasis. Possible evolutionary origins of phasmid P4 are discussed.