Haemophilus influenzae Rd lacks a stringently conserved fatty acid biosynthetic enzyme and thermal control of membrane lipid composition.

The organization of the fatty acid synthetic genes of Haemophilus influenzae Rd is remarkably similar to that of the paradigm organism, Escherichia coli K-12, except that no homologue of the E. coli fabF gene is present. This finding is unexpected, since fabF is very widely distributed among bacteria and is thought to be the generic 3-ketoacyl-acyl carrier protein (ACP) synthase active on long-chain-length substrates. However, H. influenzae Rd contains a homologue of the E. coli fabB gene, which encodes a 3-ketoacyl-ACP synthase required for unsaturated fatty acid synthesis, and it seemed possible that the H. influenzae FabB homologue might have acquired the functions of FabF. E. coli mutants lacking fabF function are unable to regulate the compositions of membrane phospholipids in response to growth temperature. We report in vivo evidence that the enzyme encoded by the H. influenzae fabB gene has properties essentially identical to those of E. coli FabB and lacks FabF activity. Therefore, H. influenzae grows without FabF function. Moreover, as predicted from studies of the E. coli fabF mutants, H. influenzae is unable to change the fatty acid compositions of its membrane phospholipids with growth temperature. We also demonstrate that the fabB gene of Vibrio cholerae El Tor N16961 does not contain a frameshift mutation as was previously reported.