TssM is expressed and secreted inside cells following infection with B. mallei [29], however, secretion occurs independently SYN-117 in vitro of T3SS3 and T6SS1 [31]. BsaN was also found to activate expression of a mTOR inhibitor review putative non-ribosomal peptide synthase (NRPS)/polyketide synthase (PKS) biosynthesis locus. The diversity of polyketides, PKSs and NRPS/PKS hybrid systems was recently reviewed by Hertweck [37]. The B. pseudomallei locus is

similar in gene content to that of a recently described plasmid encoded NRPS/PKS system in the marine bacterium Alteromonas macleodii, which was suggested to produce a bleomycin-related antibiotic Unlike A. macleodii, the gene encoding the putative bleomycin-family resistance protein (BPSL2883) is not co-localized with the NRPS/PKS gene cluster, although they are similarly regulated by BsaN (Table 1). BsaN is homologous to the Salmonella typhimurium InvF, Shigella flexneri MxiE and Tanespimycin mw the Yersinia enterocholitica YsaB transcriptional regulators [38–40]. All belong to the AraC/XylS family of transcriptional

regulators, which act in complex with a chaperone to activate their respective T3SS genes. The chaperones not only serve as cognate partners to the transcriptional activators but also pair with T3SS translocase proteins, which are secreted into the host membrane to facilitate the injection of effector proteins [41]. We currently, have no understanding of the timed mechanism that frees BicA and allows it to partner with BsaN. The

S. typhimurium chaperone SicA was shown to partition the translocase SipB and SipC, and it is sequestered by SipB [42]. Once apparatus assembly is complete, translocases are secreted and SicA is free to complex and thus activate InvF. The InvF-SicA split feedback regulatory loop, which includes positive autoregulation of invF, is conserved in Y. enterocholitica [40]. 3-mercaptopyruvate sulfurtransferase However, in S. flexneri MxiE-dependent activity is inhibited via sequestration by the T3SS substrate OspD1 when the apparatus is inactive [43]. Only when OspD1 is secreted, can MxiE partner with its chaperone IpgC to activated transcription of effector genes. Regulation by BsaN-BicA is distinct from the previously described systems. The designation of BsaN-BicA as a dual-function regulatory protein complex is illustrated by its role in activating T3SS effector and accessory genes while repressing the system’s structural and secretion components as summarized in Figure 7. BsaN was also found to suppress the transcription of 51 additional genes in the B. pseudomallei genome including those belonging to the fla1 flagellar and chemotaxis locus on chromosome 1 (Figure 1E). Fla1 is the sole flagellar system in Southeast Asian B. pseudomallei strains such as KHW, in contrast to Australian B. pseudomallei isolates which possess a complete second system encoded on chromosome 2 (Fla2) [9,44].