Relationship between cyclopropane synthetase and the formation of cyclopropane fatty acids by Proteus vulgaris grown under various respiratory conditions.

In medium saturated with oxygen, the cyclopropane synthetase (unsaturated-phospholipid methyltransferase; EC 2.1.1.16) of Proteus vulgaris was generally synthesized after the mid-exponential phase of growth. The enzyme could also be induced by rapidly limiting the oxygen supply, or by initiating respiration on nitrate or thiosulphate following an initial period of growth in a highly aerobic environment. In each of these 'step-down' situations the specific activity of cyclopropane synthetase rose to a maximum prior to the stationary phase of growth and subsequently decreased. The cyclopropane fatty acids, methylene hexadecanoic acid and methylene octadecanoic acid accumulated throughout exponential growth following the induction of the enzyme. During a 12 h period in the stationary phase there was little synthesis of either of the fatty acids, despite detectable cyclopropane synthetase activity in the cells, indicating that essentially all the fatty acid synthesis was complete prior to entering the stationary phase. When nitrate was used as a respiratory electron acceptors a twofold increase in octadecenoic acid was observed, giving rise to an increase in methylene octadecanoic acid. This increase in octadecanoic acid was not apparent in mutants unable to respire on nitrate.