Estructura y función del bacteriofitocromo del fitopatógeno Xanthomonas campestris

MALAMUD, F.; RINALDI J; FERNÁNDEZ LÓPEZ, M.; TOSACNI, A.; MROGINSKI, M.A.; GOLBAUM, F.A.; KLINKE S; VELÁZQUEZ-ESCOBAR, F.; RODRIGO SIEIRA; COLUCCIO LESKOW, F.; VOJNOV, A.A.; OTERO, L.H.; TAUM, L.; SYCZ, G.; GUDESBLAT, G.; HILDEBRANDT, P.; BONOMI, H.R.

San Miguel de Tucumán

Congreso; III Reunión de Fotobiólogos Moleculares Argentinos.; 2016

Grupo Argentino de Fotobiología

Phytochromes constitute a major superfamily of light-sensing proteins that are reversibly photoconverted between a red-absorbing (Pr) and a far-red-absorbing (Pfr) state. Bacteriophytochromes (BphPs) are found among photosynthetic and non-photosynthetic bacteria, including pathogens. To date, several BphPs have been biophysically characterized. However, it is still not fully understood how structural changes are propagated from the photosensory module to the output module during the signal transduction event. We have solved the crystal structure of the full-length BphP from the plant pathogen Xanthomonas campestris pv. campestris (XccBphP) bearing its photosensor and its complete output module, a PAS9 domain [1]. In the crystals, the protein was found to be in the Pr state whereas diffraction data together with resonance Raman spectroscopic and theoretical results indicate a ZZZssa and a ZZEssa chromophore configuration corresponding to a mixture of Pr and MetaR state, the precursor of Pfr. The XccBphP quaternary assembly reveals a head-to-head dimer in which the output module contributes to the helical dimer interface. The photosensor, which is shown to be a bathy-like BphP, is influenced in its dark reactions by the output module. Our structural analyses suggest that the photoconversion between the Pr and Pfr states in the full-length XccBphP may involve changes in the relative positioning of the output module. We then tested the hypothesis that XccBphP plays a role in bacterial pathogenesis. Our findings, show that illumination of X. campestris, prior to plant infection, attenuates bacterial virulence in an XccBphP-dependent manner. In response to light, XccBphP down-regulates xanthan exopolysaccharide production and biofilm formation, two known Xcc virulence factors. Furthermore, the XccbphP null-mutant shows enhanced virulence, similar to that of dark-adapted Xcc cultures. Stomatal aperture regulation and callose deposition, both wellestablished plant defense mechanisms against bacterial pathogens, are overridden by the XccbphP strain. Additionally, an RNA-Seq analysis reveals that far-red light or XccBphP overexpression produce genome-wide transcriptional changes, including the inhibition of several Xcc virulence systems. Our findings show that Xcc senses light through XccBphP, eliciting bacterial virulence attenuation via down-regulation of bacterial virulence factors. The capacity of XccBphP to respond to light both in vitro and in vivo was abolished by a mutation on the conserved Cys13 residue. Taken together, our work provides insight into (i) the XccBphP light sensory mechanisms at the atomic scale and (ii) its physiological role in a plant-pathogen interaction. Referencias 1- Otero LH, Klinke S, Rinaldi J, Velázquez-Escobar F, Mroginski MA, Fernández López M, Malamud F, Vojnov AA, Hildebrandt P, Goldbaum FA, Bonomi HR. J Mol Biol. 2016. doi: 10.1016/j.jmb.2016.04.012