Molecular cloning, expression, and characterization of a Ca ²⁺-dependent, membrane-associated nuclease of Mycoplasma genitalium

In this study, we identified and characterized the enzymatic properties of MG-186, a calcium-dependent Mycoplasma genitalium nuclease. MG-186 displays the hallmarks of nucleases, as indicated by its amino acid sequence similarity to other nucleases. We cloned, UGA corrected, expressed, purified, and demonstrated that recombinant MG-186 (rMG-186) exhibits nuclease activity similar to that of typical sugar-nonspecific endonucleases and exonucleases. Biochemical characterization indicated that Ca²⁺ alone enhances its activity, which was inhibited by divalent cations, such as Zn²⁺ and Mn ²⁺. Chelating agents EGTA and EDTA also inhibited nuclease activity. Mycoplasma membrane fractionation and Triton X-114 phase separation showed that MG-186 was a membrane-associated lipoprotein, and electron microscopy revealed its surface membrane location. Incubation of purified human endometrial cell nuclei with rMG-186 resulted in DNA degradation and morphological changes typical of apoptosis. Further, immunofluorescence analysis of rMG-186-treated nuclei indicated that morphological changes were linked to the disintegration of lamin and the internalization of rMG-186. Since M. genitalium has the capacity to invade eukaryotic cells and localize to the perinuclear and nuclear region of parasitized target cells, MG-186 has the potential to provide M. genitalium, which possesses the smallest genome of any self-replicating cell, with the ability to degrade host nucleic acids both as a source of nucleotide precursors for growth and for pathogenic purposes.