One means to sense and respond to environmental stimuli are two-component regulatory systems (TCSs). A modified TCS consisting of a histidine protein kinase (HPK), CorS, a response regulator (RR), CorR, and a third component, CorP, regulates COR production at the transcriptional level in P. syringae . CorP showed high similarity to CorR but lacks a DNA binding domain characteristic for RRs. Mutational analysis demonstrated that CorP is necessary for induction of COR biosynthesis , but its exact function remained to be determined. The HPK, CorS, is believed to respond to a temperature change via autophosphorylation of a conserved histidine residue, and transduces the signal to the cognate RR CorR via phosphorylation of its conserved aspartate residue .
In vitro results indicated that CorR was able to bind to the CMA and CFA biosynthetic promoter regions in a temperature- and corS-dependent manner [14,15]. Additionally, the alternative sigma factor RpoN (��54) which is required for the expression of a variety of virulence determinants and metabolic functions was shown to be essential for COR biosynthesis in P. syringae . Consequently, COR gene expression seems to be regulated by its specific TCS and by at least one global regulator.In natural settings PG4180 encounters temperature fluctuations and adapts COR gene expression accordingly. In this study, we investigated effects of a temperature shift from 28 to 18 ��C on transcription of CMA biosynthetic genes in vitro as well as in planta and on CmaB protein biosynthesis.
We also evaluated transcription of the regulatory gene corS after the temperature shift and studied whether de novo protein biosynthesis is required for transcriptional activation of COR biosynthetic genes. Taking into account that stability of mRNA contributes to mRNA levels and that mRNA stability is also influenced GSK-3 by temperature, we investigated the stability of the cma transcript at 18 and 28 ��C after inhibition of transcriptional initiation. The HPK CorS is a membrane-associated protein, which possesses a hydrophobic N-terminus comprising six transmembrane domains (TMDs) . HPKs of this structure are generally believed to sense environmental stimuli by means of their periplasmic loops. In some sensory proteins periplasmic domains for substrate binding were identified, e.g.
the nitrate binding Nit domain of the sensor histidine kinases NarX and NarQ of E. coli . Another example is the citrate binding CitAP domain in the CitA sensor kinase of Klebsiella pneumoniae . In the case of CorS, a temperature decrease results in COR biosynthesis. Whether the temperature change on its own or an additional signal activates the regulatory cascade remains to be elucidated.2.?Materials and Methods2.1. Bacterial strains, plasmids, and growth conditionsP. syringae pv. glycinea PG4180  was maintained at 28 ��C on MG agar plates .