A conditionally lethal mutant of Escherichia coli lacking phosphatidylglycerol in vivo at 42 degrees C has been previously isolated by two-stage mutagenesis (M. Nishijima and C. R. H. Raetz, J. Biol. Chem. 254:7837-7844, 1979). In the first step (designated pgsA444) the phosphatidylglycerophosphate synthetase is partially inactivated, but the resulting strain continues to make about two-thirds of the normal level of phosphatidylglycerol and is not temperature sensitive. The second lesion, termed pgsB1, causes temperature-sensitive growth and phosphatidylglycerol synthesis in strains harboring pgsA444. The pgsA locus appears to be the structural gene for the synthetase and maps near min 42. In the present study we mapped the pgsB1 mutation and characterized its interaction with pgsA444 by genetic and biochemical methods. Unexpectedly, pgsB1 was not a second lesion in the pgsA structural gene, but rather mapped at a distinct site near minute 4. P1 vir-mediated contransduction suggested the gene order pantonA-dapD-pgsB-dnaE (clockwise). Independent evidence for the genetic mapping was provided by the identification of two hybrid ColE1 plasmids (pLC26-43 and pLC34-20. L. Clarke and J. Carbon, Cell 9:91-99, 1976) which both carry pgsB+ and dnaE+. Introduction of either the pgsA+ or the pgsB+ gene (via episomes, hybrid plasmids or P1 vir transduction) suppressed the temperature sensitivity of the double mutant (pgsA444 pgsB1) and restored normal levels of phosphatidylglycerol at 42 degrees C. In addition, strains with the pgsA+ pgsB1 genotype produced a novel lipid (X) at all temperatures, whereas the double mutant (pgsA444 pgsB1) contained two unusual lipids (X and Y) after 3 h at 42 degrees C. Both X and Y are precursors of lipopolysaccharide, and introduction of pgsB+ into the double mutant caused the disappearance of X and Y. Although the biochemical basis of the pgsB1 lesion is unknown, its existence suggests a previously unrecognized link between lipopolysaccharide and phosphatidylglycerol syntheses in E. coli.