Homeoviscous adaptation.
In response to biophysical and chemical challenges such as temperature, osmolarity, pH and exogenous fatty acids, a bacterium is able to maintain the correct membrane viscousity. This compositional adaptation of membrane termed homeoviscous adaptation, is primarily controlled by the ratio of unsaturated fatty acids (UFAs) to saturated fatty acids (SFAs) in the phospholipid membrane.
In Gram-negative bacteria, this is achieved by altering the UFA:SFA ratio fed in the de novo fatty acid biosynthetic pathway involving glycerol-phosphate acyltransferases.
FabR - a transcriptional regulator.
In Escherichia coli, this is regulated by a trancriptional repressor protein, FabR which is involved in the expressions of two genes; fabA and fabB. These genes, which are essential in UFA synthesis, have binding sites located within their promoters. The binding of FabR to its cognate DNA is promoted with the presence of unsaturated acyl-acyl carrier protein (acyl-ACP) or acyl-CoA and is antagonised with the presence of saturated acyl-ACP or acyl-CoA.
DesT in Pseudomonas aeruginosa.
As a homolog to FabR, DesT does not only regulate the expression of the fabAB operon, but also the expression of the desCB operon which encodes for the oxygen-dependent acyl-CoA Δ9-desaturase. This enzyme is responsible for the conversion of SFA-CoA (16:0-or 18:0-CoA) into Δ9-UFA-CoA (16:1Δ9- or 18:1Δ9-CoA).
SFA-CoA induces the release of DesT from DNA hence, promotes the expression of desaturase while UFA-CoA acts the opposite. SFA-CoA stabilises the tense state of the DesT, unbound to DNA. UFA-CoA stabilises the relaxed state of DesT, bound to DNA. Similar to FabR, DesT have approximately equal binding affinity towards SFA-CoA and UFA-CoA suggesting that this repressor protein detects the UFA:SFA ratio rather than the concentration of an acyle-CoA ligand.
DesT-DNA-UFA Complex
This web-page is going to discuss the mechanism by which fatty acids that differ by only one double bond can modulate the binding of DesT to DNA via the determined crystal structures of DesT-UFA-CoA-DNA and DesT-SFA-CoA complexes.
