Haemophilus influenzae Rd Lacks a Stringently Conserved Fatty Acid Biosynthetic Enzyme and Thermal Control of Membrane Lipid Composition

Abstract

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.The fatty acid synthetic pathway of Escherichia coli has provided a very successful model for fatty acid synthesis in bacteria and plant chloroplasts (13, 38). Indeed, not only are the sequences of the E. coli fatty acid synthetic proteins highly conserved in other bacteria, but surprisingly, in many cases the arrangements of the genes encoding the synthetic enzymes are also conserved. As discussed elsewhere (7), the major exception to the E. coli paradigm is in the synthesis of unsaturated fatty acids. In E. coli, synthesis of the normal fatty acid content requires three enzymes, the products of the fabA, fabB, and fabF genes. FabA is the key enzyme of the classic anaerobic pathway of unsaturated fatty acid synthesis (2) and introduces the cis (or Z) double bond into a 10-carbon intermediate (Fig.

Parallel genomic evolution and metabolic interdependence in an ancient symbiosis

“…Both fabF and fabB can perform all of the elongation steps in saturated fatty acid synthesis, but perform distinct reactions in unsaturated fatty acid biosynthesis (19). Escherichia coli carrying mutations in fabF show deficiencies in the temperature control of fatty acid composition (20), and Haemophilus influenzae Rd, which lacks fabF, is unable to alter the fatty acid content of its membranes over a wide range of growth temperatures (21). It is unclear how, if at all, Sulcia and Baumannia coordinate these distinct roles in fatty acid biosynthesis.…”

Unexpected Functional Diversity among FadR Fatty Acid Transcriptional Regulatory Proteins

“…These genes have putative operators that are reasonable matches to the known fabA and fabB operators of E. coli. Indeed, because these organisms lack the generic FabF protein that catalyzes the elongation reaction of long chain fatty acid synthesis and have only FabB to catalyze these elongations (36), FadR activation of the fabB promoter may be more important than in E. coli.…”

Escherichia coli Unsaturated Fatty Acid Synthesis

“…Mass spectra were obtained as described above. Fatty acid compositions of membrane phospholipids were determined as previously described (33).…”