Sivapragasam Smitha, Deochand Dinesh K, Meariman Jacob K, Grove Anne
Department of Biological Sciences, Louisiana State University , Baton Rouge, Louisiana 70803, United States.
Biochemistry. 2017 Oct 31;56(43):5831-5843. doi: 10.1021/acs.biochem.7b00844. Epub 2017 Oct 20.
Agrobacterium fabrum induces tumor growth in susceptible plant species. The upregulation of virulence genes that occurs when the bacterium senses plant-derived compounds is enhanced by acidic pH and limiting inorganic phosphate. Nutrient starvation may also trigger the stringent response, and purine salvage is among the pathways expected to be favored under such conditions. We show here that phosphate limitation induces the stringent response, as evidenced by production of (p)ppGpp, and that the xdhCSML operon encoding the purine salvage enzyme xanthine dehydrogenase is upregulated ∼15-fold. The xdhCSML operon is under control of the TetR family transcription factor XdhR; direct binding of ppGpp to XdhR attenuates DNA binding, and the enhanced xdhCSML expression correlates with increased cellular levels of (p)ppGpp. Xanthine dehydrogenase may also divert purines away from salvage pathways to form urate, the ligand for the transcription factor PecS, which in the plant pathogen Dickeya dadantii is a key regulator of virulence gene expression. However, urate levels remain low under conditions that produce increased levels of xdhCSML expression, and neither acidic pH nor limiting phosphate results in induction of genes under control of PecS. Instead, expression of such genes is induced only by externally supplemented urate. Taken together, our data indicate that purine salvage is favored during the stringent response induced by phosphate starvation, suggesting that control of this pathway may constitute a novel approach to modulating virulence. Because bacterial purine catabolism appears to be unaffected, as evidenced by the absence of urate accumulation, we further propose that the PecS regulon is induced by only host-derived urate.
根癌农杆菌可在易感植物物种中诱导肿瘤生长。当细菌感知到植物衍生化合物时，毒力基因的上调会因酸性pH值和有限的无机磷酸盐而增强。营养饥饿也可能触发严谨反应，嘌呤补救途径是在此类条件下有望受到青睐的途径之一。我们在此表明，磷酸盐限制会诱导严谨反应，如（p）ppGpp的产生所证明，并且编码嘌呤补救酶黄嘌呤脱氢酶的xdhCSML操纵子上调了约15倍。xdhCSML操纵子受TetR家族转录因子XdhR的控制；ppGpp与XdhR的直接结合会减弱DNA结合，并且增强的xdhCSML表达与（p）ppGpp细胞水平的增加相关。黄嘌呤脱氢酶还可能使嘌呤从补救途径转向形成尿酸盐，尿酸盐是转录因子PecS的配体，在植物病原体达旦氏果胶杆菌中，PecS是毒力基因表达的关键调节因子。然而，在产生增加的xdhCSML表达水平的条件下，尿酸盐水平仍然很低，酸性pH值和有限的磷酸盐都不会导致PecS控制下的基因诱导。相反，此类基因的表达仅由外部补充的尿酸盐诱导。综上所述，我们的数据表明，在磷酸盐饥饿诱导的严谨反应期间，嘌呤补救受到青睐，这表明控制该途径可能构成一种调节毒力的新方法。由于细菌嘌呤分解代谢似乎未受影响，如尿酸盐积累的缺乏所证明，我们进一步提出，PecS调控子仅由宿主衍生的尿酸盐诱导。
