Abstract
The plant pathogenic bacterium Pseudomonas syringae PG4180 synthesizes high levels of the phytotoxin coronatine (COR) at the virulence-promoting temperature of 18 °C, but negligible amounts at 28 °C. Temperature-dependent COR gene expression is regulated by a modified two-component system, consisting of a response regulator, CorR, the histidine protein kinase CorS, and a third component, termed CorP. We analyzed at transcriptional and translational levels the expression of corS and the cma operon involved in COR biosynthesis after a temperature downshift from 28 to 18 °C. Expression of cma was induced within 20 min and increased steadily whereas corS expression was only slightly temperature-dependent. Accumulation of CmaB correlated with accumulation of cma mRNA. However, cma transcription was suppressed by inhibition of de novo protein biosynthesis. A transcriptional fusion of the cma promoter to a promoterless egfp gene was used to monitor the cma expression in vitro and in planta. A steady induction of cma::egfp by temperature downshift was observed in both environments. The results indicate that PG4180 responds to a temperature decrease with COR gene expression. However, COR gene expression and protein biosynthesis increased steadily, possibly reflecting adaptation to long-term rather than rapid temperature changes.
Highlights
In order to survive, bacteria constantly have to adapt to changing environmental conditions.Environmental stimuli can serve pathogenic bacteria as signals for expression of virulence factors, for example, temperature often serves as a trigger for the induction of virulence gene expression
COR is a secondary metabolite and not essential for bacterial survival at low temperatures [8]. By this adaptation to long-term temperature decreases the bacteria may presumably induce the energy-consuming COR biosynthesis only under conditions which favour the infection of the host plant
COR is synthesized from two precursor molecules, coronafacic acid (CFA) and coronamic acid (CMA)
Summary
Bacteria constantly have to adapt to changing environmental conditions. Environmental stimuli can serve pathogenic bacteria as signals for expression of virulence factors, for example, temperature often serves as a trigger for the induction of virulence gene expression. While temperatures of warm-blooded hosts (37-41 °C) are important signals for the induction of virulence gene expression in humans and animal pathogens [1], plant pathogens generally express their virulence genes in response to lower temperatures [2,3,4]. The later organisms adjust their virulence gene expression in response to diurnal and seasonal temperature fluctuations. COR is a non-host specific phytotoxin, which is an important virulence factor of several pathovars of Pseudomonas syringae, precisely pv. Biosynthesis of COR is positively regulated by low temperature in
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