Chilling-sensitive, post-transcriptional regulation of a plant fatty acid desaturase expressed in yeast.

Plants respond to chilling exposure by increasing the relative proportion of polyunsaturated fatty acids in their lipids. However, unlike the response in many other organisms, plant fatty acid desaturase genes are typically not upregulated during this process. We expressed the Brassica napus FAD3 gene, which encodes an enzyme for synthesis of linolenic acid, in Saccharomyces cerevisiae and observed a temperature-dependent increase in linolenic acid production at cooler growth temperatures. Untransformed yeast cells, however, responded to cooler temperatures primarily by shortening fatty acid chains, even when polyunsaturated fatty acids were supplied in the growth media. Measurement of the steady-state levels of Fad3 protein in transformed yeast revealed an 8.5-fold increase in steady-state amount of desaturase enzyme when cells were cultivated at cooler temperatures. The increase was not due to changes in transcriptional activity, since Northern hybridization revealed no appreciable changes in abundance of FAD3 transcripts at cooler temperatures. Taken together, the results suggest that the increase in linolenic acid content in cells containing Fad3 was not due to enhanced physiological demand for polyunsaturated fatty acids by yeast, but rather a cold-inducible, post-transcriptional increase in steady-state amount of plant desaturase enzyme. Implications for plant adaptation to chilling are discussed.