Abstract
MetE and MetH are two distinct enzymes that catalyze a similar biochemical reaction during the last step of methionine biosynthesis, MetH being a cobalamin-dependent enzyme whereas MetE activity is cobalamin-independent. In this work, we show that the last step of methionine synthesis in the plant pathogen Ralstonia solanacearum is under the transcriptional control of the master pathogenicity regulator HrpG. This control is exerted essentially on metE expression through the intermediate regulator MetR. Expression of metE is strongly and specifically induced in the presence of plant cells in a hrpG- and metR-dependent manner. metE and metR mutants are not auxotrophic for methionine and not affected for growth inside the plant but produce significantly reduced disease symptoms on tomato whereas disruption of metH has no impact on pathogenicity. The finding that the pathogen preferentially induces metE expression rather than metH in the presence of plant cells is indicative of a probable metabolic adaptation to physiological host conditions since this induction of metE occurs in an environment in which cobalamin, the required co-factor for MetH, is absent. It also shows that MetE and MetH are not functionally redundant and are deployed during specific stages of the bacteria lifecycle, the expression of metE and metH being controlled by multiple and distinct signals.
Highlights
Ralstonia solanacearum is a Gram-negative soil-borne b-proteobacterium which is the causal agent of bacterial wilt, one of the most devastating bacterial plant diseases in the world
Expression of metH was modulated by HrpG as the bgalactosidase activity of the metH::lacZ fusion was decreased by a 2.5-fold factor in the hrpG mutant compared to its expression in the wild type (Figure 2B). These results showed that HrpG controls expression of genes involved in the last step of methionine biosynthesis: this control is exerted independently from HrpB and appears tighter on metE than on metHab
This study uncovers a direct link between basal metabolism and pathogenicity in R. solanacearum, revealing that methionine metabolism is activated once the bacteria are in the presence of plant cells and is genetically connected through the HrpG regulon to other essential pathogenicity determinants such as the T3SS
Summary
Ralstonia solanacearum is a Gram-negative soil-borne b-proteobacterium which is the causal agent of bacterial wilt, one of the most devastating bacterial plant diseases in the world. R. solanacearum can rapidly multiply in the xylem up to very high cell densities, leading to wilting symptoms and plant death. As soon as it enters the host root tissue, R. solanacearum has to face hostile environmental conditions due to plant defence reactions and to limited nutritional resources in the plant apoplasm. A key player of this regulatory system is HrpG, which directly or indirectly controls the transcriptional induction of more than 350 genes in the presence of plant cells, including those directing the synthesis of the Type III Secretion System (T3SS) and effector substrates which is essential to pathogenesis [5,6,7]. HrpG integrates multiple environmental signals and controls, beyond the T3SS, many other bacterial functions that promote disease (reviewed in [8])
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