Role of the Bacillus subtilis fatty acid desaturase in membrane adaptationduring cold shock

In our attempt to understand the cold shock response of Bacillus subtilis, we report on the role of the B. subtilis fatty acid desaturase (FA-D) Des d uring membrane adaptation to low temperatures and demonstrate its importanc e during cold shock. A des null mutant was constructed and analysed in comp arison with its parental strain. Growth studies and large-scale comparative fatty acid (FA) analysis revealed a severe cold-sensitive phenotype of the des deletion mutant during the absence of isoleucine and showed that four unsaturated fatty acid (UFA) species differing in length, branching pattern and position of the double bond are synthesized in B. subtilis JH642 but n ot in the des null mutant. Apart from the lack of UFA synthesis, the FA-D d eletion strain showed a dramatically altered saturated fatty acid (SFA) pro file at the onset of the stationary growth phase in the presence of exogeno us isoleucine sources. Expression of des integrated in trans at the amyE lo cus of the des deletion strain not only cured the cold-sensitive phenotype observed for the des mutant but allowed much better growth than in strain J H642 after a shift from 37 degreesC to 15 degreesC. These results show that , during cold shock adaptation, des expression can completely replace the i soleucine-dependent, long-term, FA branching adaptation mechanism. We concl ude that the crucial aspect in cold adaptation of the cytoplasmic membrane is not its specific molecular composition but rather its physical status in terms of its fluidity.