Oxygen-dependent low-temperature A 1 2 ( n 6 )-desaturase induction and alteration of fatty acid composition in Acantharnoeba caste / / anii

The influence of dissolved oxygen on the synthesis and activity of A12desaturase in Acantharnoeba castellanii was investigated. A decline in oxygen concentration during batch growth at 30 “C was correlated with a decline in the degree of cellular fatty acid unsaturation. Chil l ing of early-stationaryphase cultures to 15 OC led to increased dissolved oxygen levels (from < 1 pM to 305 pM) and increased fatty acid unsaturation, which has been shown previously [Avery, 5. V., Harwood, J. L. & Lloyd, D. (1994) Microbiology 140, 2423-24311 to be due mainly to A12-desaturase induction. In contrast, chilling of mid-exponential-phase cultures, where the dissolved oxygen concentration prior to chilling was high (> 160 pM), gave no change in cellular fatty acid unsaturation. Measurement of [l-14C]acetate incorporation by oxygen-limited A. castellanii revealed that labelling of the A12-desaturase product, linoleate (18: 2), increased with oxygen concentration. Microsomal levels of the A12desaturase enzyme were found to increase by up to 10-fold during aeration of A. castellanii cultures; a transient elevation in oxygen was sufficient to induce Al2-desaturase synthesis that was still fully detectable 1 h later. In addition, the activity of pre-existing A1 2-desaturase, measured in isolated microsomal membranes, increased by up to fivefold with increases in the oxygen concentration of assay mixtures. These results demonstrate for the first t ime that (i) oxygen availability alone can regulate de novo A12-desaturase synthesis in A. castellanii, and that (ii) oxygen can limit the activity of preexisting A 1 2-desaturase. These responses can occur independently of temperature changes.

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