SITE-DIRECTED MUTAGENESIS OF HISTIDINE-RESIDUES IN THE DELTA-12 ACYL-LIPID DESATURASE OF SYNECHOCYSTIS

In the cyanobacterium Synechocystis sp, PCC 6803, there are four acyl-lipid desaturases that are, respectively, specific to the Delta 6, Delta 9, Delta 12 and omega 3 positions of fatty acids. The desA gene for the Delta 12 acyl-lipid desaturase was modified by site-directed mutagenesis, such that four of the histidine residues that are conserved in the four desaturases and one histidine residue that is not conserved were replaced by arginine, and the mutated desA genes were overexpressed in Escherichia coli. All of these mutations eliminated the Delta 12 desaturase activity. These results demonstrate that the five histidine residues are essential for the activity of the Delta 12 desaturase, perhaps by providing the ligands for the catalytic Fe center.