Widespread Head-to-Head Hydrocarbon Biosynthesis in Bacteria and Role of OleA

摘要

Previous studies identified the oleABCD genes involved in head-to-head olefinic hydrocarbon biosynthesis. The present study more fully defined the OleABCD protein families within the thiolase, α/β-hydrolase, AMP-dependent ligase/synthase, and short-chain dehydrogenase superfamilies, respectively. Only 0.1 to 1% of each superfamily represents likely Ole proteins. Sequence analysis based on structural alignments and gene context was used to identify highly likely ole genes. Selected microorganisms from the phyla Verucomicrobia , Planctomyces , Chloroflexi , Proteobacteria , and Actinobacteria were tested experimentally and shown to produce long-chain olefinic hydrocarbons. However, different species from the same genera sometimes lack the ole genes and fail to produce olefinic hydrocarbons. Overall, only 1.9% of 3,558 genomes analyzed showed clear evidence for containing ole genes. The type of olefins produced by different bacteria differed greatly with respect to the number of carbon-carbon double bonds. The greatest number of organisms surveyed biosynthesized a single long-chain olefin, 3,6,9,12,15,19,22,25,28-hentriacontanonaene, that contains nine double bonds. Xanthomonas campestris produced the greatest number of distinct olefin products, 15 compounds ranging in length from C_(28) to C_(31) and containing one to three double bonds. The type of long-chain product formed was shown to be dependent on the oleA gene in experiments with Shewanella oneidensis MR-1 ole gene deletion mutants containing native or heterologous oleA genes expressed in trans . A strain deleted in oleABCD and containing oleA in trans produced only ketones. Based on these observations, it was proposed that OleA catalyzes a nondecarboxylative thiolytic condensation of fatty acyl chains to generate a β-ketoacyl intermediate that can decarboxylate spontaneously to generate ketones.