Endpoint distribution for deletions into imm lambda region forming p lambda CM replicons: phage lambda gene rex affects plasmid establishment.

For the p lambda CM family of lambda-derived self-encapsidating plasmids, the rexB gene product facilitates plasmid establishment following injection into a new host cell. Temperature-stable chloramphenicol resistance (CmR at 40 degrees C) conferred by low-multiplicity infection with lambda::Tn9 cI857 lysates (Tn9 sites tested: 22.60 or 24.08 kb, in the b region, or 28.41 kb, in int) is usually due to a lambda::Tn9 plasmid (p lambda CM) formed by a deletion penetrating the lambda immunity region. These grow either as plasmids in the absence of, or lytic phages in the presence of N function supplied by a host such as lambda cI857 delta H1 lysogen MS1449. The 'groplaque' (plaque-shaped growth spot) assay, which selects for CmR growth in an MS1449 lawn at 32 degrees C after an initial plaquing period at 37 degrees C, reveals two distinguishable classes of p lambda CM isolates. All variants whose deletions extend into or beyond rexB give rise to visible CmR growth only after the temperature shift to 32 degrees C, and thus produce a hollow-centered 'donut' type of groplaque. In contrast, 16 out of 17 variants whose deletions fall short of rexB produce 'solid' groplaques which appear before the temperature shift. Tests of T4rII phage exclusion show the exceptional 17th variant to be Rex-, confirming the identification of rex as the lambda component whose loss results in the 'donut' groplaque morphology. More specific physiological tests showed that in the absence of Rex the establishment of a newly injected p lambda CM plasmid becomes temperature-sensitive (ts), while plasmid maintenance remains unaffected. This indicates that the role of Rex in plasmid survival is confined to the early stages of transduction, where it might either assist plasmid replication or retard host replication, to help the plasmid replicon achieve a copy number sufficient for stable transmission.