Fig. 2
From: The plasmid-mediated evolution of the mycobacterial ESX (Type VII) secretion systems
Model of ESX evolution based on plasmid-mediated duplication and evolution. The ancestral ESX-4 gene cluster evolved from the WXG-FtsK cluster via the incorporation of additional genes, eccB, eccD, mycP and rv3446c. ESX-4 was duplicated into plasmid DNA, into which additional ESX genes, eccA, eccE, espI, espG, pe and ppe, were incorporated. The plasmid ancestor (ESX-PAN) was reinserted into the genomes of various Mycolata, generating ESX-4EVOL. Continuous evolution generated the operonic structure of the plasmid ESX gene cluster. Divergent evolution of the plasmid ESX generated several plasmid ESX (ESX-P1, -P3, -P2’, -P2 and -P5) which were inserted into the mycobacterial genome to generate ESX-3, ESX-1, ESX-2 and ESX-5. An earlier version of ESX-P1 was inserted into the genomes of some actinomycetes as ESX-1AN and a precursor of ESX-P2’ was inserted into the M. tusciae genome as ESX-2AN. Red arrows represent genome insertions