## 9 Transcriptional and Post-Transcriptional Regulation of _ltrpG_ and _ltrpX_
Intriguingly, flagellar and T4 SS assembly genes, which are usually associated with epiphybit fitness, are negatively regulated by _Xanthomonas_ RsmA/CsrA [114; 41], suggesting that RsmA/CsrA potentially serves as a molecular switch to distinguish between the epiphy and endophytic lifestyle of the bacteria. Regulation of HrPG at a posttranslational level was found to be mediated by phosphorylation and protein stability. Similarly to other OmpR-family transcriptional regulators, HrPG harbors a putative phosphorylation site at position D60 [41; 42; 62]. The ratio of phosphorylated HrPG of _Xcci_ is increased when bacteria are incubated in plant-mimicking media compared with rich media [41], supporting that the regulator is activated by phosphorylation. The exact signal and the sensor kinase(s) that promote HrPG phosphorylation were unknown for a significantly long period. Two hybrid studies conducted in _Xcci_ and _Xcci_ identified two-component sensor kinases that displayed strong interaction with HrPG [62; 115]. These sensor kinases, _Xcci_ XAC3683 and _Xcc_ XC_3670 (HpaS), do not share significant homology or genomic localization with each other. _Xcci_ XAC3683 was not  
Figure 3: Schematic representation of direct transcriptional and post-transcriptional regulation of HrPG and HrPX. Red “-1” represents negative transcriptional regulation of _hrpG_ and _hrpX_ based on transcriptional expression and promoter binding analyses. The Blue arrow represents positive transcriptional regulation of _hrpX_ based on transcriptional expression and promoter-binding analyses. Black “-1” and arrows represent negative and positive post-transcriptional regulation of HrPG, respectively, based on biochemical and functional analyses. Black circled “P” represents protein phosphorylation. Data represent information derived from _Xcci_rub subsp. _citri_ (_Xcci_), _X. oryzae_ pv. _orange_ (_Xoo_), and _X. campstris_ _pv. _amapstris_ (_Xcc_). subjected to any further study and its significance to the function of HrpG is unknown. The _hpqA_ null mutant is deficient in virulence, causes attenuated HR response in resistant plants, and has reduced expression of _hrp_ gene in plant-mimicking media and _in planta_[62]. Further analyses identified that HrpG phosphorylation in plant-mimicking media is dependent on HpaS, suggesting that HrpG is directly phosphorylated by HpaS in _Xcc_[62] (Figures 2 and 3). The protein stability of HrpG is regulated by the Lon protease in _Xcci_[116] (Figure 2). Deletion of _lon_ resulted in elevated _hrp_ transcript accumulation, hypervulence in host plants, and stronger HR induction in non-host plants [116]. The observed phenotypes correlate with increased stability of the HrpG protein, indicating that Lon negatively regulates HrpG [116] (Figure 3). _In vitro_ and _in vivo_ analyses revealed that HrpG is degraded by Lon through recognition of an N-terminal site in HrpG, which promotes its cleavage by the protease [116]. Genetic and biochemical data show that Lon activity is controlled by phosphorylation-mediated deactivation [116]. The environmental cues controlling Lon phosphorylation or the adjusted kinase that phosphorylate Lon are yet to be identified.