The Bacillus subtilis desaturase:: a model to understand phospholipid modification and temperature sensing

Most fatty acid desaturases are members of a large superfamily of integral membrane, O-2-dependent, iron-containing enzymes that insert double bonds into previously synthesized fatty acyl chains. The cold shock-induced, membrane-bound desaturase from Bacillus subtilis (Delta 5-Des) uses existing phospholipids as substrates to introduce a cis-double bond at the fifth position of the fatty acyl chain. While essentially no three-dimensional structural information is available for these difficult-to-purify enzymes, experimental analysis of the topology of Delta 5-Des has provided a model that might be extended to most acyl-lipid desaturases. In addition, studies of the cold-induced expression of Delta 5-Des led to the identification of a two-component system composed of a membrane-associated kinase, DesK, and a transcriptional regulator, DesR, which stringently controls the transcription of the des gene, coding for the desaturase. A model for sensing and transduction of low-temperature signals has emerged from our results, which we discuss in the context of transcriptional regulation of membrane lipid fluidity homeostasis.