Two aerobic pathways for the formation of unsaturated fatty acids in Pseudomonas aeruginosa

The double bond in anaerobic unsaturated fatty acid (UFA) biosynthesis is introduced by the FabA dehydratase/isomerase of the bacterial type II fatty acid biosynthetic pathway. A Delta fabA mutant of Pseudomonas aeruginosa grew aerobically, but required a UFA supplement for anaerobic growth. Wild-type cells produced 18:1 Delta 11 as the principal UFA, whereas the Delta fabA strain produced only 16:1 Delta 9. The double bond in the 16:1 Delta 9 was introduced after phospholipid formation and was localized in the sn-2 position. Two predicted membrane proteins, DesA and DesB, possessed the conserved histidine clusters characteristic of fatty acid desaturases. The Delta fabA Delta desA double mutant required exogenous fatty acids for growth but the Delta fabAdesB double mutant did not. Exogenous stearate was converted to 18:1 Delta 9 and supported the growth of Delta fabA Delta desA double mutant. A Delta fabA Delta desAdesB triple mutant was unable to desaturate exogenous stearate and was an UFA auxotroph. We detected a 2.5-fold increase in desA expression in Delta fabA mutants, whereas desB expression was derepressed by the deletion of the gene encoding a transcriptional repressor DesT. These data add two aerobic desaturases to the enzymes used for fatty acid metabolism in proteobacteria: DesA, a 2-position phospholipid Delta 9-desaturase that supplements the anaerobic FabA pathway, and DesB, an inducible acyl-CoA Delta 9-desaturase whose expression is repressed by DesT.