OXYGEN-DEPENDENT LOW-TEMPERATURE DELTA-12 (N6)-DESATURASE INDUCTION AND ALTERATION OF FATTY-ACID COMPOSITION IN ACANTHAMOEBA-CASTELLANII

The influence of dissolved oxygen on the synthesis and activity of Del ta 12-desaturase in Acanthamoeba castellanii was investigated. A decli ne in oxygen concentration during batch growth at 30 degrees C was cor related with a decline in the degree of cellular fatty acid unsaturati on, Chilling of early-stationary-phase cultures to 15 degrees C led to increased dissolved oxygen levels (from < 1 mu M to 305 mu M) and inc reased fatty acid unsaturation, which has been shown previously [Avery , S. V., Harwood, J. L. & Lloyd, D. (1994) Microbiology 140, 2423-2431 ] to be due mainly to Delta 12-desaturase induction. In contrast, chil ling of mid-exponential-phase cultures, where the dissolved oxygen con centration prior to chilling was high (> 160 mu M), gave no change in cellular fatty acid unsaturation, Measurement of [1-C-14]acetate incor poration by oxygen-limited A. castellanii revealed that labelling of t he Delta 12-desaturase product, linoleate (18:2), increased with oxyge n concentration. Microsomal levels of the Delta 12-desaturase enzyme w ere found to increase by up to 10-fold during aeration of A. castellan ii cultures; a transient elevation in oxygen was sufficient to induce Delta 12-desaturase synthesis that was still fully detectable 1 h late r. In addition, the activity of pre-existing Delta 12-desaturase, meas ured in isolated microsomal membranes, increased by up to fivefold wit h increases in the oxygen concentration of assay mixtures. These resul ts demonstrate for the first time that (i) oxygen availability alone c an regulate de novo Delta 12-desaturase synthesis in A. castellanii, a nd that (ii) oxygen can limit the activity of preexisting Delta 12-des aturase. These responses can occur independently of temperature change s.