Suppression of the Lethal Effect of Acidic-Phospholipid Deficiency in Escherichia Coli by Bacillus Subtilis Chromosomal Locus Ypop

An acidic-phospholipid deficiency caused by the pgsA3 allele that encodes a defective phosphatidylglycerophosphate synthase in Escherichia coli is lethal. The only known mutations that suppress this lethality fully have been related to the major outer-membrane lipoprotein. We isolated a Bacillus subtilis chromosomal locus that suppresses the lethality when harbored in a low copy-number plasmid, without restoring the synthase activity or phospholipid composition to normal. The locus was first recognized to suppress the conditional lethality of E. coli YA5512 (pgsA3) that harbored an unidentified mutation(s), allowing its growth in LB medium but not in media of lower osmolarities. The locus was then found to suppress the lethality of pgsA3 in wild-type E. coli W3110. This locus, named ypoP in the database, had 37% nucleotide identity with the E. coli mprA gene, but the amplification of mprA had no suppressive effect. Plasmid pPOP1 containing ypoP completely prevented the decrease in the amount of a porin protein, OmpF, in the outer membrane and also cell mucoidy caused by pgsA3. The mechanisms underlying these unusual effects are discussed in relation to a putative stress signal(s) generated by the acidic-phospholipid deficiency.