Novel Type-IVC Secretion System in Streptococcus Figure 1. Genetic organization of T4SS. For A. tumefaciens, gene names B = virB and D4 = virD4. For L. pneumophila, upper-case gene names = dot and lower-case gene names = icm. Genes in orange correspondents to periplasmic lytic transglycosylase, in green ATPase, in yellow T pilus, in red channel subunit across inner membrane, in blue channel subunit across outer membrane, in grey no T4SS genes or genes with the unclear function. doi:10.1371/journal.pone.0046390.g001 not be caused by a mutation, but could perhaps be caused by several rounds of DNA acquisition from other strains. Our previous experiment proved that a GI with Type-IVC secretion system can spontaneously excise to form an extrachromosomal circular product and laterally transfer to another strain [23]. Considering the proven mobility of Type-IVC secretion system, we believe that Horizontal Transfers (HT) between species brought about the acquisition of Type-IVC secretion system in Streptococcus. Figure 3B further shows that virB/D genes in Type- IVC secretion system have higher similarity in the same species than that between two species, suggesting that the movement of GIs caused by HT occurs more easily within strains of a species than that between species. Discussion Minimal Components of Type-IVC Secretion System In Gram-negative bacteria, Type-IVA secretion system usually consists of 12 components to work together just as the figure 2 shown. Thorough investigations about functions of these 12 Figure 2. Hypothetical model for Type-IVC secretion system. The colored parts are the components of the Type-IVC secretion system system. The grey parts are objects present in the canonical T4SS of Gram-negative strains but lost in the Gram-positive Type-IVC secretion system, e.g., the outer membrane, VirB2, and VirB5. doi:10.1371/journal.pone.0046390.g002 PLOS ONE | www.plosone.org 4 October 2012 | Volume 7 | Issue 10 | e46390