Prophage λ induction caused by mini-F plasmid genes

Abstract

When bacterial cells harboring a temperaturesensitive replication plasmid, which carries the particular ccd segment of a mini-F plasmid, are transferred to 42°C, cell division is inhibited after incubation for an appropriate time. The inhibition occurs, when the copy number of the plasmid decreases to become critically low, about one per cell (Ogura and Hiraga 1983 b). In λ phage lysogens carrying this type of plasmid, the prophage is induced in a small portion of the cell population under the same conditions, in addition to the inhibition of cell division in most of cells. The prophage induction, but not the inhibition of normal cell division, depends on normal recA function. Both induction of prophage and inhibition of cell division are suppressed by the simultaneous presence of a replication proficient plasmid carrying the ccdA gene. We discuss molecular mechanisms of the ccd function that couples host cell division to plasmid proliferation and induces the prophage. Additionally, we propose a hypothesis that the ccd mechanism of F plasmid contributes to indirect induction of prophage λ by an F plasmid damaged by UV-irradiation and then introduced into a lysogen via conjugation.