Possible evolutionary scenario for the MTBC PPE38 gene region. Analysis of fast growing mycobacterial species and the M. avium complex indicates that the homologues of Rv2345 and glyS have been in close proximity for a long evolutionary period and that the insertion of homologues to PPE38 and Rv2348c between these genes was also a relatively early event (a, b). The most recent common ancestor of the M. marinum/M. ulcerans and MTBC lineages is hypothesised to have comprised a single esx/esx/PPE38 gene cluster (black box) located between the plcABC (yellow) and PPE39/40 (grey) gene regions (c). Duplication of esx/esx/PPE38 resulted in a genotype that is retained by M. marinum (d). The genotype of the ancestral MTBC species (e) shows an additional deletion of the esx/esx gene pair between plcA and PPE38 (red box).