Summary
The virulence plasmid‐encoded type III secretion system of Shigella flexneri
consists of the Mxi–Spa secretion apparatus, secreted proteins IpaA–D
and IpgD involved in entry of bacteria into epithelial cells, cytoplasmic chaperones
IpgC and IpgE and 15 other secreted proteins of unknown function, including VirA
and members of the IpaH family. The activity of the Mxi–Spa apparatus is regulated
by external signals, and transcription of virA and ipaH genes is specifically
induced in conditions of active secretion. We present genetic evidence that regulation
of these genes involves both MxiE, the transcriptional activator of the AraC family
encoded by the mxi operon, and IpgC, the chaperone for IpaB and IpaC. We also
show that together MxiE and IpgC are sufficient to activate virA and ipaH9.8
promoters in Escherichia coli. In S. flexneri, increasing the expression of IpgC led to a concomitant increase in IpaH production in conditions of non‐secretion. This suggests that the activity of secretion is sensed by the presence of free IpgC, which acts as a coactivator to allow MxiE to activate transcription at its target promoters.
SummaryBacteria of Shigella spp. are responsible for shigellosis in humans and use a type III secretion (TTS) system to enter epithelial cells and trigger apoptosis in macrophages. Transit of translocator and effector proteins through the TTS apparatus is activated upon contact of bacteria with host cells. Transcription of ª ª ª ª 15 genes encoding effectors is regulated by the TTS apparatus activity and controlled by MxiE, an AraC family activator, and its coactivator IpgC, the chaperone of IpaB and IpaC translocators. Using a genetic screen, we identified ospD1 as a gene whose product negatively controls expression of genes regulated by secretion activity. OspD1 associates with the chaperone Spa15 and the activator MxiE and acts as an antiactivator until it is secreted. The mechanism regulating transcription in response to secretion activity involves an activator (MxiE), an anti-activator (OspD1), a co-anti-activator (Spa15), a coactivator (IpgC) and two anti-coactivators (IpaB and IpaC) whose alternative and mutually exclusive interactions are controlled by the duration of the TTS apparatus activity.
Shigella, the etiological agent of human bacillary dysentery, invades the colonic epithelium where it induces an intense inflammatory response. Entry of Shigella into epithelial cells involves a type III secretion machinery, encoded by the mxi and spa operons, and the IpaA-D secreted proteins. In this study, we have identified secreted proteins of 46 and 60 kDa as the products of virA and ipaH9.8, respectively, the latter being a member of the ipaH multigene family. Inactivation of virA did not affect entry into epithelial cells. Using lacZ transcriptional fusions, we found that transcription of virA and four ipaH genes, but not that of the ipaBCDA and mxi operons, was markedly increased during growth in the presence of Congo red and in an ipaD mutant, two conditions in which secretion through the Mxi-Spa machinery is enhanced. Transcription of the virA and ipaH genes was also transiently activated upon entry into epithelial cells. These results suggest that transcription of the virA and ipaH genes is regulated by the type III secretion machinery and that a regulatory cascade differentially controls transcription of genes encoding secreted proteins, some of which, like virA, are not required for entry.
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