Generation of Mycoplasma hominis gene-targeted mutants by targeting-induced local lesions in genomes (TILLING)

Background Mycoplasma hominis is a human urogenital pathogen involved in gynaecological, neonatal and extra-genital infections. However, no versatile genetic tools are currently available to study the pathogenicity of this bacterium. Targeting-Induced Local Lesions IN Genomes (TILLING) is a reverse-genetic method that combines point mutations induced by chemical mutagenesis with a DNA screening technique. We used ethyl methanesulfonate (EMS) that introduces C-G to T-A transition mutations to generate a library of M. hominis mutants. As a proof of concept, mutagenized organisms were screened for mutations in two target genes previously associated with the mycoplasma pathogenicity, the vaa gene encoding an adhesin lipoprotein and the oppA gene encoding the main ecto ATPase of the bacterium. The resulting mutants were evaluated using functional assays, an adhesion to HeLa cell assay for vaa -mutants and an ATPase activity test for oppA -mutants. Results A 1200-clone library was generated by exposing M. hominis PG21 to 9 mg/mL EMS for 3 h. To identify mutants of interest, targeted gene fragments were amplified, heat-denatured, slowly reannealed and digested with the mismatch-specific endonuclease ENDO1. If multiple alleles were present in the PCR amplicons, these alleles formed heteroduplexes during reannealing that were specifically cleaved by ENDO1 at mismatching positions. A total of four vaa -mutants and two oppA -mutants harbouring missense mutations were obtained and fully sequenced. Zero to eight additional mutations were identified in the genomes of each mutant. The vaa -mutants were tested for adhesion to immobilized HeLa cells but their adhesion was not significantly different from the adhesion of M. hominis PG21. One of the two oppA -mutants that were tested for ATPase activity presented a higher affinity for its ATP substrate than the parental strain. Conclusion For the first time, we demonstrated that M. hominis gene-targeted mutants could be successfully obtained using this TILLING strategy. In the absence of robust genetic tools for studying M. hominis , the TILLING strategy that can target any gene of the genome could help to elucidate gene functions and to better understand the pathogenesis of this human pathogenic species.