Cover photograph (Copyright © 2004, American Society for Microbiology. All Rights Reserved.): The Des pathway. The output of signal transduction is governed by the order of membrane lipids that switch the activity of the DesK sensor protein (yellow) between kinase- and phosphatase-biased forms. An increase in the order of membrane lipids may cause a conformational change that enables DesK to phosphorylate its cognate regulator DesR (blue). The phosphorylated form of DesR activates the transcription of the des gene, coding for the desaturase (red). The desaturase produced decreases the ordering of membrane lipids by introducing double bonds into the phospholipid acyl chains. The restoration of membrane fluidity favors the hydrolysis of phosphorylated DesR by DesK and the activation of des ends, and the adaptation cycle is completed. (See related article on page 6681.)