PE_PGRS3: a new player inMycobacterium tuberculosispathogenesis

TheM. tuberculosis(Mtb) genome contains around 60 pe_pgrs genes, whose role and function remain elusive. In this study, two PE_PGRS proteins with high sequence homology were selected and investigated (PE_PGRS3 and PE_PGRS4), with PE_PGRS3 characterized by the presence of a C-terminal domain rich in arginine. Interestingly, full-length PE_PGRS3 protein is expressed by Mtb strains but not by other MTBC subspecies causing disease in animals. A gene reporter system was developed to investigate inM. smegmatis(Msm) the expression pattern of these genes. Fluorescence microscopy, FACS and transcriptional analysis indicated that the two genes are differentially regulated, with pe_pgrs3 but not pe_pgrs4 being expressed only when mycobacteria are cultivated in low inorganic phosphate (iPhos). Expression of pe_pgrs3 in low iPhos correlated with the upregulation of relA in Msm recombinant strains and Mtb, suggesting that pe_pgrs3 is involved in the stringent response. Overexpression of the PE_PGRS3, and of its functional deletion mutant (PE_PGRS3ΔCt), in Msm were obtained by expressing these genes under control of hbhA promoter. Interestingly, Msm strains overexpressing PE_PGRS3 showed enhanced ability to entry in macrophages and epithelial cells compared to Msm expressing PE_PGRS3ΔCt or Msm parental strain. No differences in the ability of these strains to survive intracellularly were measured. These results provide new insights on the role of PE_PGRS3 in TB pathogenesis.