Characterisation of the fumarase FumC and of its interaction with the essential histidine kinase PdhS at the old pole of Brucella abortus

Mignolet, Johann [UNamur]

Recently the bacterial pathogen Brucella abortus has been shown to asymmetrically divide in a reminiscent way compared to the leguminous symbiont Sinorhizobium meliloti and to the differentiation paradigm Caulobacter crescentus, two bacteria phylogenetically related to B. abortus. In C. crescentus, several proteins belonging to the well-known two-components signal transduction systems take a large part in the establishment of cellular asymmetry and in the differentiation process undergone by the flagellated cell type to become a stalked cell type. Strikingly many of these proteins have a homolog in B. abortus and S. meliloti, as predicted from genomes analysis, which suggests the possibility that a conserved network could operate an equivalent differentiation event in these two bacteria. Furthermore these two host-associated bacteria encode in their genome at least one supplementary protein protagonist. Therefore we focused our work on a main divergence in B. abortus and C. crescentus signalling network components namely the histidine kinase PdhS. We first demonstrated that pdhS gene is essential and that its protein product is asymmetrically distributed to focus at the old pole of B. abortus. This molecular behaviour strengthens the differentiation hypothesis in B. abortus. We subsequently identified the class II fumarate hydratase FumC (one of the key enzymes in the tricarboxylic acids cycle) as a partner of PdhS that specifically interacts with the "sensory" domain of PdhS. Logically FumC colocalizes with PdhS at the old pole of B. abortus while the B. abortus class I fumarate hydratase FumA that does not interact with PdhS remains scattered throughout the cytoplasm. Surprisingly we also discovered that the positional information encompassed in FumC is altered in its homologs in C. crescentus and S. meliloti. To sum up, our data suggest that the aforementioned bacteria that belong to the same taxonomic class share the common characteristic to differentiate albeit they have developed diverse mechanistic specificities along the evolution.