Soutenance de doctorat de Sara MOUTACHARRIF (Equipe CRP)

Étude de la régulation post-transcriptionnelle chez Dickeya dadantii : le cas de la régulation par l’excludon impliquant des régulateurs de la virulence

Originally described in tropical regions, bacteria of the genus Dickeya are now considered emerging pathogens in Europe. They cause soft rot disease in a wide range of hosts, including crops of major agronomic importance such as potato and rice. Understanding the mechanisms governing the regulation of their virulence factors is an essential step toward developing new control strategies. Although many transcriptional regulators have been characterized, the infection cycle remains only partially understood. In particular, the dynamics of the transitions between different stages of infection are still poorly elucidated. Post-transcriptional regulation, by its ability to rapidly and finely tune gene expression, is likely to play a key role in orchestrating these transitions.

Among the different post-transcriptional regulatory mechanisms are excludons, defined as genomic loci where two adjacent genes transcribed in opposite orientations share an overlapping region, one transcript acting as an antisense RNA that negatively regulates its complementary partner. In this work, we first examined the prevalence of excludons involving the fifteen major virulence regulators in Dickeya dadantii. Analysis of their genomic context revealed that almost all of them are associated with excludon-like organizations. These overlaps mainly involve convergently transcribed genes, most often between virulence regulators and metabolic or stress-response genes. We then investigated in detail the excludon formed by the pecS gene, encoding a central transcriptional regulator of virulence, and the adjacent argG gene, involved in arginine metabolism. We demonstrated that the regulation of virulence genes by PecS depends on the concentration of arginine in the environment, and that this regulation relies on the overlap between the pecS and argG transcripts. These results show that excludon organization can directly connect two distinct biological functions, here virulence and metabolism, and dynamically modulate gene expression in response to environmental conditions. Given the high number of transcription factors whose genes are organized in excludons, this regulatory mechanism may be widespread and allow Dickeya to rapidly integrate physiological signals into the control of virulence.

In conclusion, this work highlights an original mechanism of post-transcriptional regulation that enables a phytopathogenic bacterium to finely adapt the expression of its virulence factors to environmental cues. Beyond D. dadantii, it paves the way for a broader understanding of the role of overlapping transcripts in other bacteria sharing the same genetic organization.