FUNCTION OF THE ESCHERICHIA-COLI MSBB GENE, A MULTICOPY SUPPRESSOR OFHTRB KNOCKOUTS, IN THE ACYLATION OF LIPID-A - ACYLATION BY MSBB FOLLOWS LAURATE INCORPORATION BY HTRB

Overexpression of the Escherichia coli msbB gene on high copy plasmids suppresses the temperature-sensitive growth associated with mutations in the htrB gene, htrB encodes the lauroyl transferase of lipid A bio synthesis that acylates the intermediate (Kdo)(2)-lipid IVA (Brozek, I t. A. and Raetz, C. R. H. (1990) J. Biol. Chem, 265, 15410-15417). Sin ce msbB displays 27.5% identity and 42.2% similarity to htrB, me explo red the possibility that msbB encodes a related acyltransferase, In co ntrast to htrB, extracts of strains with insertion mutations in msbB a re not defective in transferring laurate from lauroyl acyl carrier pro tein to (Kdo)(2)-lipid IVA. However, extracts of msbB mutants do not e fficiently acylate the product formed by HtrB, designated (IZdo)(2)-(l auroyl)-lipid IVA. Extracts of strains harboring msbB(+) bearing plasm ids acylate (Kdo)(2)-(lauroyl)-lipid IVA very rapidly compared with wi ld type, We solubilized and partially purified MsbB from an overproduc ing strain, lacking HtrB, MsbB transfers myristate or laurate, activat ed on ACP, to (Kdo)(2)-(lauroyl)-lipid IVA. Decanoyl, palmitoyl, palmi toleoyl, and (R)-3-hydroxymyristoyl-ACP are poor acyl donors, MsbB acy lates (Kdo)(2)-(lauroyl)-lipid IVA about 100 times faster than (Kdo)(2 )-lipid IVA. The slow, but measurable, rate whereby MsbB acts on (Kdo) (2)-lipid IVA may explain why overexpression of MsbB suppresses the te mperature-sensitive phenotype of htrB mutations, Presumably, the acylo xyacyl group generated by excess MsbB substitutes for the one normally formed by HtrB.