A Regulatory Mechanism for the Balanced Synthesis of Membrane Phospholipid Species in<i>Escherichia coli</i>

The mechanism that assures the balanced synthesis of zwitterionic (phosphatidylethanolamine) and acidic phospholipids (phosphatidylglycerol and cardiolipin) in Escherichia coli has been examined by genetically manipulating the two enzymes at the biosynthetic branch point, i.e., phosphatidylglycerophosphate synthase, encoded by pgsA, and phosphatidylserine synthase, encoded by pssA. A mutant in which the most part of the pssA gene was replaced with a drug resistance gene lacked phosphatidylserine synthase and phosphatidylethanolamine and required divalent metal ions for growth, as did a previously reported insertion-inactivated pssA mutant. When this mutant harbored a plasmid containing a Bacillus subtilis gene that encodes membrane-bound phosphatidylserine synthase, the phosphatidylethanolamine content was dependent on its activity, in contrast to that with the soluble E. coli counterpart. A defective mutation, pgsA3, caused reductions not only in acidic-phospholipid synthesis but also in phosphatidylethanolamine synthesis, despite the normal level of phosphatidylserine synthase activity. These results, together with previous observations, indicate that phosphatidylserine synthesis requires the membrane-associated form of phosphatidylserine synthase, which is related to the membrane-levels of acidic phospholipids, thus yielding balanced compositions of zwitterionic and acidic phospholipids.