# Innovation and Application of the Type III Secretion System Inhibitors in Plant Pathogenic Bacteria
## 2 Regulation of T3 SS in _D. dadantii_
### Transcriptional Regulators Control T3 SS  
The T3 SS and flagellum are evolutionarily closely related due to their similarities in structure, function, and sequences of their main components [68; 69; 70; 71]. In _P. syringae_, flagellin could be translocated into plant cells by the T3 SS and induce immune responses [72]. Genetically, the master regulator of the flagellar assembly genes FlhDC has been shown to be essential for the activation of T3 SS genes in _D. dadantti_ and _Pectobacterium carotovorum_[51; 73; 74; 75; 76]. A detailed genetic study conducted by [PERSON] et al. demonstrated that _D. dadantti_ FlhDC transcriptionally initiates the expression of _ecpC_, which regulates the T3 SS through the c-di-GMP-mediated RpoN-HrpL pathway (Figure 1) [50; 51]. Interestingly, the homolog of EcpC, named YhJH, in _E. coli_ is also positively regulated by FlhDC. However, different from _E. coli_, the alternative sigma factor (\(\sigma^{28}\)) FlA, which is under the control of FlhDC, is not required for the activation of _ecpC_ via FlhDC in _D. dadantti_[51; 77]. Additionally, since the consensus FlhDC binding sequence, AA(C/T)G(C/G)N\({}_{2\text{-}3}\)AAATA(A/G)CG [78; 79], is not present in the promoter region of _ecpC_, the activation of _ecpC_ by FlhDC might be indirect. FlhDC and YcgR, a PilZ domain protein, have also been shown to be involved in the regulation of T3 SS through the RsmA/RsmB system (Figure 1) [51] and the function of YcgR has been shown to be activated by the binding to c-di-GMP [80; 81]. However, in vitro studies demonstrated that their contributions to T3 SS are not significant compared with the FlhDC-EcpC-RpoN-HrpL pathway [51].  
The sigma factor RpoS is the master regulator for the stress response in bacteria [82]. In _E. coli_, RpoS is degraded by the ClpXP protease with the assistance of the recognition factor RssB [83; 84; 85]. RpoS from _P. carotovorum_ has been reported to affect the production of Pel, T3 SS gene _hrpN_ expression, and virulence in plant through its regulation on RsmA [86]. Subsequent studies from _D. dadantti_ and _E. amylovora_ showed similar findings with a more detailed regulatory mechanism proposed in _D. dadantti_: The ClpXP-RssB-RpoS regulatory cascade controls the expression of _rsmA_, which post-transcriptionally affects HrpL; RpoS represses _hrpL_ transcription without affecting HrpS or RpoN (Figure 1) [87; 88], suggesting that a novel RpoS-HrpL pathway might exist in _D. dadantti_.  
SlyA belongs to the member of the SlyA/MarR family transcriptional regulator [89]. In _D. dadanti_, SlyA was first characterized as a Pel regulator, and the deletion of _slyA_ significantly reduced disease symptoms in planta [90]. Since SlyA has been reported to regulate the expression of T3 SS in _Salmonella enterica serovar_ Typhimurium [91], the role of SlyA homologue in controlling _D. dadanti_ T3 SS was investigated. [PERSON] and colleagues reported that SlyA negatively regulates HrpL via two pathways: It upregulates the expression of _rsmA_ and downregulates the transcription of _hrpS_ that is independent of the TCSTS HrpX/HrpY (Figure 1). Interestingly, despite its negative impact on _hrpL_, SlyA positively regulates the expression of _hrp_ regulon genes, such as _hrpA_ and _hrpN_, in parallel with HrpL (Figure 1) [92], suggesting that multiple factors might be involved in the transcriptional regulation of T3 SS genes in _D. dadantti_. Indeed, PecS, another MarR family transcriptional regulator [93; 94], and PecT, a LysR family transcriptional regulator [37], have been shown to repress the transcription of _hrpN_ (Figure 1) [59]. In vitro DNA-protein binding and DNase I footprinting analyses confirmed that PecS directly interacts with the promoter of _hrpN_ to repress its transcription [59]. The mechanism of PecT-mediated regulation on _hrpN_ remains unclear. Nevertheless, it will be interesting to know how and under what circumstances these transcriptional regulators modulate T3 SS regulon gene expression in a cooperative or rather competitive manner.