Cyclophilin (the product of the ppiB gene) and the trigger factor (the product of the tig gene) are the only cytosolic peptidyl-prolyl
cis-trans isomerases that are known in Bacillus subtilis. Both
enzymes catalyze the in vitro refolding of
ribonuclease T1, a reaction that is limited in rate by a prolyl cis/trans isomerization. The efficiency of
cyclophilin as a folding catalyst is only modest with a kcat/KM value of 3.8 x 10(4) M-1 s-1, but the trigger factor shows an almost 40-fold higher specific activity with a kcat/KM value of 1.4 x 10(6) M-1 s-1. This high catalytic activity originates from the tight binding to the
protein substrate as reflected in both the low KM value of 0.5 microM and in the strong inhibition of the trigger factor by unfolded
proteins. By use of a protein-folding assay, the concentrations of
cyclophilin and the trigger factor in the cytosol of B. subtilis could be determined as 26 and 35 microM, respectively. Together they account for the entire folding activity that is detectable in
crude extracts of wild-type B. subtilis cells. The genes encoding
cyclophilin and the trigger factor in the B. subtilis chromosome were disrupted individually and simultaneously. Even in combination, these disruptions had no effect on cell viability in rich medium or under several stress conditions, such as heat, osmotic, or oxidative stress. However, in poor medium and, in particular, in the absence of
amino acids, the growth of the double mutant strain was strongly decelerated, indicating that the prolyl
isomerases become essential for growth under
starvation conditions. It is not yet known whether this function relates to the catalysis of the
proline-limited folding of essential
proteins.