Doctor of Philosophy / Plant Pathology / Frank F. White / Rice and wheat are two major crops that suffer losses from the diseases of bacterial blight and bacterial leaf streak, which are caused by Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas translucens pv. undulosa (Xtu), respectively. Transcriptional-Activator Like (TAL) effectors, a special family of type III effector proteins from Xanthomonas, have been demonstrated as critical virulence factors that act by inducing corresponding susceptibility (S) genes in several disease complexes of plants. In this study, I analyzed the contributions of TAL effectors from Xoo and Xtu to virulence and in modulating host gene expression to enhance susceptibility. Specifically, the TalC effector from the African Xoo strain AXO1947 was identified as a critical virulence factor, which functions by promoting expression of the gene OsSWEET14 in rice. TalC is interchangeable with other major TAL effectors from Asian strains of Xoo on the basis of functional complementation. The TAL effector PthXo2 from the Asian Xoo strain JXO1 is a major virulence factor and contains 21.5 repeats in the central repetitive region that targets OsSWEET13 in indica rice varieties but not in japonica rice varieties. A one repeat deletion in the PthXo2 effector enabled effector specificity to switch from indica rice to japonica rice. TAL effector genes from a genomic analysis of the Xtu strain XT4699 and related strains were characterized with regards to their involvement in virulence and the modulation of host gene expression in the Chinese Spring wheat cultivar. The identification of TAL effectors with virulence contributions and their target S genes is important for understanding the virulence mechanisms of Xanthomonas bacteria and promises to provide new strategies for disease control.

Streptococcus pneumoniae est la cause principale de pneumonies, otites, méningites et septicémies. La transformation génétique naturelle constitue l’élément clé de son adaptation aux changements environnementaux. Elle s’effectue par intégration d’ADN d’origine externe dans le chromosome de la bactérie, et a lieu pendant un état physiologique particulier appelé compétence.Mon travail de thèse a consisté à étudier les acteurs principaux de la régulation de la compétence (ComD, ComE) et les protéines impliquées dans la prise en charge, le traitement de l’ADN transformant et la recombinaison (DprA, RecA). J’ai notamment résolu la structure du facteur de transcription ComE par cristallographie aux rayons X, et réalisé une étude fonctionnelle de sa fixation sur un de ses promoteurs. Les résultats obtenus ont permis de proposer un mécanisme selon lequel la dimérisation induite par la phosphorylation de ComE, couplée à sa fixation sur la séquence promotrice d’ADN, provoquerait une courbure de l’ADN. Cette courbure permettrait la fixation de l’ARN polymérase, activant ainsi la transcription des gènes nécessaires à la mise en place de la compétence. / Streptococcus pneumoniae is the leading cause of community-acquired infections worldwide. The natural genetic transformation is the key to its adaptation to environmental changes. It takes place with the integration in its chromosome of exogenous DNA, during a physiological state called competence.During my thesis I have focused on the main actors of competence regulation (ComD, ComE) and on proteins involved in exogenous DNA processing and recombination (DprA, RecA). In particular, I have solved the structure of the transcriptional activator ComE by X-ray crystallography, and carried out a functional study of its binding to its promoter. The results obtained allowed us to propose a mechanism regarding the transcriptional activation by ComE of the genes necessary for the set up of the competence : the phosphorylation-induced dimerization, coupled to the binding of ComE to its DNA promoter, would curve the DNA and allow the binding of the RNA polymerase.