Figure 1. Schematic representation of the gene clusters encoding core components and substrates of T7S-like systems in M. tuberculosis (ESX-1), S. aureus (Ess) and B. subtilis (Ess).

The nomenclature of ESX-1 genes is that proposed by Bitter et al [15]. Note that, for simplicity, several ESX-1 secretion-associated protein (esp) genes, located immediately or far upstream of M. tuberculosis eccA1, are not represented. Genes consistently described as essential (or important) for secretion of cognate WXG100 proteins, or of other specific substrates, are marked with the letter “E” [3], [4], [16], [17] (see text for B. subtilis Ess). Genes coding for products sharing conserved domains are depicted with the same color code, whereas those specific of each system are colored in white. Features of conserved gene products are indicated below (TMD stands for transmembrane domain). EssB and YukC harbor a pseudokinase domain [18], [19].