Mutants of Bacillus species isolated on the basis of protonophore resistance are deficient in fatty acid desaturase activity

The fatty acid desaturase activity in cell extracts of Bacillus subtilis was characterized and found to be O₂ dependent, NADH dependent, and cyanide sensitive. In cell fractionation studies, only 10% of the desaturase activity was recovered in the membrane fraction; the addition of cytosolic factors, which by themselves were devoid of activity, restored membrane activity to the level found in the unfractionated cell extracts. NADH was preferred over NADPH as an electron donor, and palmitoyl-coenzyme A was used preferentially over stearoyl-coenzyme A as the straight-chain fatty acid substrate. An increase in desaturase activity was observed when either the growth or the assay temperature was lowered from 37 to 20°C, although the assay temperature appeared to be the more important parameter. Three protonophore-resistant mutants of B. subtilis and a comparable mutant of Bacillus megaterium had been found to possess reduced levels of unsaturated fatty acids in their membrane phospholipids; their protonophore resistance was abolished when grown in the presence of an unsaturated fatty acid supplement. All of these strains were found to be either significantly deficient in or totally lacking desaturase activity in comparison with their wild-type parent strains. Full, protonophore-sensitive revertants of the mutants had levels of desaturase activity comparable to those of the wild-type. Temperature-sensitive revertants of two of the mutants, which grew at 32°C but not at 26°C in the presence of protonophore, exhibited desaturase activity comparable to that of the wild type at 26°C but lacked activity at 32°C. These results indicate that the biochemical basis for protonophore resistance in these Bacillus mutants is a fatty acid desaturase deficiency.