Response of Agrobacterium tumefaciens to host and bacterial quorum signals

Pathogens detect the presence of their hosts by host-released chemical signals or host-derived physical changes, which leads to induction of genes involved in virulence and suppression of host defense. Agrobacterium tumefaciens detects phenolic compounds, monosaccharides, and acidic pH released from plant wounds and induces vir genes responsible for T-DNA transfer that causes tumor formation in plants. Plant tumors produce opines, which are consumed exclusively by the bacterial cells harboring the same type of Ti plasmid as that which caused the tumor. Opines also cause the transfer of the Ti plasmids by inducing the expression of quorum sensing regulator TraR. Induction of TraR at high population density leads to the production and detection of the quorum sensing signal N-(3-oxooctanoyl)-homoserine lactone (OOHL) and this quorum sensing activates tra/trb genes required for the transfer of the Ti plasmid to other bacteria.; I compared the transcript profiles of A. tumefaciens by using microarray chips after growing them in the presence or absence of the phenolic compound acetosyringone (AS) released from plant wounds to identify the complete sets of vir genes. Unexpectedly, I found that all the genes on the Ti plasmid showed slightly induced level of expression when normalized with those on other replicons. This led to the finding that VirG directly activates the expression of repABC operon upon AS induction and this activation leads to a copy number increase of the Ti plasmid.; I also compared the transcript profiles of A. tumefaciens grown with or without overexpression of TraR to identify the complete TraR regulon. We observed unexpectedly weak induction of traAFBH operon, which led us to study the autorepression of traAFBH and traCDG operons. The autorepression of the divergent traAFBH and traCDG operons required traA, traC, and traD. The same genes were also required for the efficient processing at oriT, suggesting that formation of the relaxosome is required for both processes. We also found a strong cis-preference of TraA activity in both processes.