Interaction of the ΦHSIC virus with its host:: Lysogeny or pseudolysogeny?

The marine phage Phi HSIC has been previously reported to enter into a lysogenic relationship with its host, HSIC, identified as Listonella pelagia, This phage produces a variety of plaques on its host, including turbid and haloed plaques, from which lysogens were previously isolated. These lysogens were unstable during long-term storage at -80 degrees C and were lost, When HSIC was reinfected with phage Phi HSIC, pseudolysogen-like interactions between the phage and its host were observed. The cells (termed HSIC-2 or HSIC-2e) produced high viral titers (10(11) ml(-1)) in the absence of inoculating phage and yet reached culture densities of nearly 10(9) ml(-1). Prophages were not induced by mitomycin C or the polyaromatic hydrocarbon naphthalene in cells harboring such infections, However, such cells were homoimmune to superinfection, Colonies hybridized strongly with a gene probe from a 100-bp fragment of the Phi HSIC genome, while the host did not. Analysis of chromosomal DNA preparations suggested the presence of a chromosomally integrated prophage, Phage adsorption experiments suggested that HSIC-2 was adsorption impaired. Because of the chromosomal prophage integration and homoimmunity, we interpret these results to indicate that Phi HSIC establishes a lysogenic relationship with its host that involves an extremely high level of spontaneous induction. This could be caused by a weak repressor of phage production. Additionally, poor phage adsorption of HSIC-2 compared to the wild type probably helped maintain this pseudolysogen-like relationship. In many ways, pseudolysogenic phage-host interactions may provide a paradigm for phage-host interactions in the marine environment.