MODULATIONS IN LIPID-A AND PHOSPHOLIPID BIOSYNTHESIS PATHWAYS INFLUENCE OUTER-MEMBRANE PROTEIN ASSEMBLY IN ESCHERICHIA-COLI K-12

The assembly defect of a mutant outer membrane protein, OmpF315, can b e corrected by suppressor mutations that lower lipopolysaccharide (LPS ) levels and indirectly elevate phospholipid levels. One such assembly suppressor mutation, asmB1, is an allele of IpxC (envA) whose product catalyses the first rate-limiting step in the lipid A (LPS) biosynthe sis pathway. Besides reducing LPS levels, asmB1 confers sensitivity to MacConkey medium. A mutation, sabA1, that reverses the MacConkey sens itivity phenotype of asmB1 maps within fabZ (whose product is needed f or phospholipid synthesis from a precursor) is also required for lipid A synthesis. In addition to reversing MacConkey sensitivity, the sabA 1 mutation reverses the OmpF315 assembly suppression phenotype of asmB 1. These results show that OmpF315 assembly suppression by asmB1, whic h is achieved by lowering LPS levels, can be averted by a subsequent a berration in phospholipid synthesis at a point where the biosynthetic pathways for these two lipid molecules split. OmpF315 assembly suppres sion can also be achieved in an asmB(+) background where FabZ expressi on is increased. The data obtained in this study provide genetic evide nce that elevated phospholipid levels and/or phospholipid to LPS ratio s are necessary for assembly suppression.