Transcriptional and biochemical characterization of two Azotobacter vinelandii FKBP family members

Peptidyl-prolyl cis/trans isomerases (PPIases, EC: 5.2.1.8), a class of enzymes that catalyse the rate-limiting step of the cis/trans isomerization in protein folding, are divided into three structurally unrelated families: cyclophilins, FK506-binding proteins (FKBPs), and parvulins. Two recombinant FKBPs from the soil nitrogen-fixing bacterium Azotobacter vinelandii, designated as AvfkbX and AvfkbB, have been purified and their peptidyl-prolyl cis/trans isomerase activity against Suc-Ala-Xaa-Pro-Phe-pNA synthetic peptides characterised. The substrate specificity of both enzymes is typical for bacterial FKBPs, with Suc-Ala-Phe-Pro-Phe-pNA being the most rapidly catalysed substrate by AvfkbX and Suc-Ala-Leu-Pro-Phe-pNA by AvfkbB. Both FKBPs display chaperone activity as well in the citrate synthase thermal aggregation assay. Furthermore, using real-time RT-qPCR, we demonstrated that both genes were expressed during the exponential growth phase on glucose minimal medium, while their expression declined dramatically during the stationary growth phase as well as when the growth medium was supplied exogenously with ammonium.