Functions antagonistic to plasmid formation by lambda N- chromosomes

It was concluded in the preceding paper that lambda N- cI- genomes probably failed to form plasmids within infected Escherichia coli cells because they leakily express functions that act to destabilize the plasmid state. This prediction was investigated by examining the effect upon plasmid-forming ability of the loss of possible anti-plasmid functions. The loss of Ter function was found to allow long-term plasmid formation, although the efficiency of initial plasmid formation and the heritable stability without selection were low. The combined loss of the int, red and gam gene functions also promoted plasmid growth, although the absence of Ter lambda was necessary. In contrast, the presence of Ter80 (due to an h80 substitution) did not prevent plasmid formation when the int, red and gam genes were absent, indicating that Ter80 does not attack the closed-circular form of the lambda chromosome. The combined loss of the ter, int, red and gam gene functions facilitated fully efficient inheritance of the lambda N- cI- plasmids in the absence of selection, although the efficiency of initial plasmid formation remained low. However, cells harbouring such plasmids suffered a decline in viability, indicating that the plasmids expressed a function (or more than one) that acts to debilitate the host cells--presumably an effect that is increased with this genotype because the modified lambda N- cI- plasmids are inherently more stable. The possible involvement of the lambda S and kil functions in destabilization is discussed.