Sequence and inactivation of the pss gene of Escherichia coli. Phosphatidylethanolamine may not be essential for cell viability.

Phosphatidylethanolamine is the only zwitterionic phospholipid in Escherichia coli and accounts for 70-80% of the total glycerophospholipids of this organism. To investigate the function of phosphatidylethanolamine in E. coli, we constructed an inactivated allele (pss93::kan) of the gene encoding the phosphatidylserine synthase which catalyzes the committed step to the synthesis of phosphatidylethanolamine. Growth of this mutant was dependent on a plasmid-borne copy of the wild type gene. After curing the mutant of the wild type gene, growth stopped when the content of phosphatidylethanolamine reached 30% of the total phospholipid. Divalent metal ions at millimolar concentrations suppressed the growth phenotype of the mutant in the following order of efficiency: Ca2+ greater than Mg2+ greater than Sr2+. Although phosphatidylserine synthase activity was not detectable, phosphatidylethanolamine was still present at 0.007% of the total phospholipid after growth for many generations in rich medium containing 20 mM Mg2+. The remainder of the phospholipid was primarily phosphatidylglycerol and cardiolipin with no other unique phosphate-containing chloroform-soluble material present. The phospholipid to protein ratio and the fatty acid composition were very similar to the parental strain. The broad divalent metal ion auxotrophy brought about by the lack of phosphatidylethanolamine suggests a primarily structural role for this phospholipid in E. coli.