This article describes a simple and robust laboratory exercise on the regulation of membrane unsaturated fatty acid composition in bacteria by a decrease in growth temperature. We take advantage of the well characterized Des pathway of "Bacillus subtilis", composed of a [delta]5-desaturase (encoded by the "des" gene) and the canonical two-component system DesK-DesR, to study the transcriptional regulation of "des" during cold shock. Students analyze the expression of a reporter transcriptional fusion between the "des" promoter and the bacterial "lacZ" gene in a wild-type "B. subtilis" strain and in "des" or "desK-desR" mutants grown under different culture conditions. Measurements of [beta]-galactosidase activity allow them to investigate how the Des pathway works and to assess the role of each component of this regulatory system. (Contains 3 figures and 1 table.)