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Title: Regulatory mechanisms of exoribonuclease PNPase and regulatory small RNA on T3SS of dickeya dadantii

item ZENG, QUAN - University Of Wisconsin
item Ibekwe, Abasiofiok - Mark
item BIDDLE, EULANDRIA - University Of Wisconsin
item YANG, CHING - University Of Wisconsin

Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2010
Publication Date: 10/1/2010
Citation: Zeng, Q., Ibekwe, A.M., Biddle, E., Yang, C.H. 2010. Regulatory mechanisms of exoribonuclease PNPase and regulatory small RNA on T3SS of dickeya dadantii. Molecular Plant-Microbe Interactions. 23(10):1345-1355.

Interpretive Summary: Type tree secretion systems (T3SS) are tools in some bacteria that provide them with the ability to infect higher organisms. Defects in T3SSs may therefore render the bacterium non-pathogenic. Diseases caused by T3SS-bacteria such as Escherichia coli (food poisoning), and the plant pathogens Dickeya dadantii infect humans and plants. These bacteria have very complex enzyme systems within the T3SSs that allows them to deliver protein across membranes to cause diseases in plants or humans. In this study, one of the enzymes that have a role in causing diseases was constructed to study the effects of mutation on T3SS gene expression. Our study showed that the enzyme in Dickeya dadantii plays an essential regulatory role in the expression of T3SS genes, but not the key regulatory role. The findings from this study may provide researchers with new tools in understanding how some plant pathogens may interaction with human pathogens under certain environmental conditions.

Technical Abstract: The type III secretion system (T3SS) is an essential virulence factor for many bacterial pathogens. Polynucleotide phosphorylase (PNPase) is one of the major exoribonucleases in bacteria and plays important roles in mRNA degradation, tRNA processing, and small RNA (sRNA) turnover. In this study, we showed that PNPase downregulates the transcription of T3SS structural and effector genes of the phytopathogenic bacterium Dickeya dadantii. This negative regulation of T3SS by PNPase occurs by repressing the expression of hrpL, encoding a master regulator of T3SS in D. dadantii. By reducing rpoN mRNA stability, PNPase downregulates the transcription of hrpL, which leads to a reduction in T3SS gene expression. Moreover, we have found that PNPase downregulates T3SS by decreasing hrpL mRNA stability. RsmB, a regulatory sRNA, enhances hrpL mRNA stability in D. dadantii. Our results suggest that PNPase decreases the amount of functional RsmB transcripts that could result in reduction of hrpL mRNA stability. In addition, bistable gene expression (differential expression of a single gene that creates two distinct subpopulations) of hrpA, hrpN, and dspE was observed in D. dadantii under in vitro conditions. Although PNPase regulates the proportion of cells in the high state and the low state of T3SS gene expression, it appears that PNPase is not the key switch that triggers the bistable expression patterns of T3SS genes.