| Research Abstract |
The expression of the chloroplast FAD8 ω-3 desaturase gene changes in response to a change in ambient temperature, whereas the expression of another chloroplast ω-3 desaturase FAD7, the isozyme of FAD8 desaturase, is not affected by temperature. With the Arabidopsis fad7 mutant, as the activity of the FAD7 desaturase is deficient, only the activity of the FAD8 desaturase can be monitored for the chloroplast ω-3 desaturase, thereby making it possible to clearly recognize the temperature dependency of its enzyme acivity. The expression of the FAD8 desaturase is therefore switched on and off by a difference of as little as a few ℃, either side of 25℃. Consequently, it can be predicted that this temperature regulation operates by a rather different mechanism from the expression of temperature-dependent genes found so far, such as the HSP genes. We constructed a series of chimeric genes, created from both the FAD7 and FAD8 genes that encode the isozymes of chloroplast ω-3 desaturase, and introduced them into the Arabidopsis fad7fad8 double mutant. Analyses of these transgenic plants show that the temperature-dependent expression of the FAD8 gene is due not to the 5' flanking region, including the promoter region and the untranslated region, but to the exon/intron structure that is inherent to the FAD8 gene. It therefore seems unlikely that FAD8 gene expression is simply regulated in the transcriptional levels, as in the bacterial desaturase genes. In the root tissues of wheat, although the levels of 18:3 increase markedly at low temperatures, in accordance with the increased accumulation of the endoplasmic Reticulum ω-3 fatty acid desaturase protein, the mRNA level of the desaturase gene TaFAD3 hardly changes at all. This suggests that the increased level of 18:3 at low temperatures is regulated under the translational or post-translational level.
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