PemK Toxin of Bacillus anthracis Is a Ribonuclease

Abstract

Bacillus anthracis genome harbors a toxin-antitoxin (TA) module encoding pemI (antitoxin) and pemK (toxin). This study describes the rPemK as a potent ribonuclease with a preference for pyrimidines (C/U), which is consistent with our previous study that demonstrated it as a translational attenuator. The in silico structural modeling of the PemK in conjunction with the site-directed mutagenesis confirmed the role of His-59 and Glu-78 as an acid-base couple in mediating the ribonuclease activity. The rPemK is shown to form a complex with the rPemI, which is in line with its function as a TA module. This rPemI- rPemK complex becomes catalytically inactive when both the proteins interact in a molar stoichiometry of 1. The rPemI dis- plays vulnerability to proteolysis but attains conformational sta- bility only upon rPemK interaction. The pemI-pemK transcript is shown to be up-regulated upon stress induction with a con- comitant increase in the amount of PemK and a decline in the PemI levels, establishing the role of these modules in stress. The artificial perturbation of TA interaction could unleash the toxin, executing bacterial cell death. Toward this end, synthetic pep- tides are designed to disrupt the TA interaction. The peptides are shown to be effective in abrogating TA interaction in micro- molar range in vitro. This approach can be harnessed as a poten- tial antibacterial strategy against anthrax in the future.