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

Plants respond to chilling exposure by increasing the relative proportion o f polyunsaturated fatty acids in their lipids. However, unlike the response in many other organisms, plant fatty acid desaturase genes are typically n ot upregulated during this process. We expressed the Brassica napus FADS ge ne, which encodes an enzyme for synthesis of linolenic acid, in Saccharomyc es cerevisiae and observed a temperature-dependent increase in linolenic ac id production at cooler growth temperatures. Untransformed yeast cells, how ever, responded to cooler temperatures primarily by shortening fatty acid c hains, even when polyunsaturated fatty acids were supplied in the growth me dia. Measurement of the steady-state levels of Fad3 protein in transformed yeast revealed an 8.5-fold increase in steady-state amount of desaturase en zyme when cells were cultivated at cooler temperatures. The increase was no t 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 lino lenic acid content in cells containing Fad3 was not due to enhanced physiol ogical demand for polyunsaturated fatty acids by yeast, but rather a cold-i nducible, post-transcriptional increase in steady-state amount of plant des aturase enzyme. Implications for plant adaptation to chilling are discussed . (C) 2001 Academic Press.