Defective plasmid partition in ftsH mutants of Escherichia coli

FtsH is an ATP-dependent protease that is essential for cell viability in E scherichia coli. The essential function of FtsH is to maintain the proper b alance of biosynthesis of major membrane components, lipopolysaccharide and phospholipids. F plasmid uses a partitioning system and is localized at sp ecific cell positions, which may be related to the cell envelope, to ensure accurate partitioning. We have examined the effects of ftsH mutations on t he maintenance of a mini-F plasmid, and have found that temperature-sensiti ve ftsH mutants are defective in mini-F plasmid partition, but not replicat ion, at permissive temperature for cell growth. A significant fraction of r eplicated plasmid molecules tend to localize close together on one side of the cell, which may result in failure to pass the plasmid to one of the two daughter cells upon cell division. By contrast, an ftsH null mutant carryi ng the suppressor mutation sfhC did not affect partitioning of the plasmid. The sfhC mutation also suppressed defective maintenance in temperature-sen sitive ftsH mutants. Using this new phenotype caused by ftsH mutations, we also isolated a new temperature-sensitive ftsH mutant. Mutations in ftsH ca use an increase in the lipopolysaccharide/ phospholipid ratio due to stabil ization of the lpxC gene product, which is involved in lipopolysaccharide s ynthesis and is a substrate for proteolysis by the FtsH protease. It is lik ely that altered membrane structure affects the localization or activity of a putative plasmid partitioning apparatus located at positions equivalent to 1/4 and 3/4 of the cell length.