Antarctic psychrotolerant, Nostoc sp. (SO-36), when grown at 25 degrees C and then shifted to 10 degrees C, showed an increase in the tri-unsaturated fatty acid [C-18:3(9,C-12,(15))] at the expense of mono- [C-18:1(9)] and di-unsaturated [C-18:2(9,C-12)] fatty acids. These results indicate that the activities of the enzymes DesA and DesB are up-regulated, when cultures were grown at 10 degrees C or shifted to 10 degrees C from 25 'C. However, RT-PCR studies indicated a constitutive expression of desA, desB, desC, and desC2 genes when cultures grown at 25 degrees C were shifted to 10 degrees C. This constitutive expression of des genes is in contrast to that observed in mesophilic cyanobacteria, in which desA and desB are transcriptionally up-regulated in response to lowering of growth temperature. (c) 2007 Elsevier Inc. All rights reserved.