Both authors approved the final manuscript.”
“Background Pseudomonas syringae pv. phaseolicola is a pathogenic bacterium, that produces a disease in beans (Phaseolus vulgaris L.) known as “”Halo Blight”". This disease affects both leaves and pods, and is responsible for major field crop losses in temperate areas. Disease symptoms are typically water-soaked lesions surrounded Erismodegib by a chlorotic zone or halo. This halo is due to the action of a non-host specific toxin known as NSC23766 cell line phaseolotoxin [Nδ(N'-sulfodiaminophosphinyl)-ornithyl-alanyl-homoarginine],

which is the major virulence factor of the pathogen and a key component in the development of the disease [1–3]. Phaseolotoxin acts as a reversible inhibitor of the enzyme ornithine carbamoyltransferase (OCTase; EC2.1.3.3) that catalyzes the conversion of ornithine to citruline in the arginine biosynthesis pathway [4, 5]. The consequence of OCTase inhibition is blockage of arginine biosynthesis resulting in death of host cells. The production of check details phaseolotoxin by P. syringae pv. phaseolicola is regulated by temperature, being optimally produced at 18°C-20°C, while at 28°C (the optimal growth temperature for this bacterium) the toxin is not detected [6, 7]. Nevertheless, other factors such as plant signals and carbon sources have also been suggested as inducers of phaseolotoxin synthesis [8, 9]. Our group reported the sequence of a chromosomal

region of P. syringae pv. phaseolicola NPS3121, which contains genes involved in phaseolotoxin synthesis. This region, known as the “”Pht cluster”", includes 23 genes organized in five transcriptional units: two monocistronic, argK and phtL, and three polycistronic, a large operon from phtA to phtK, with an internal promoter capable of driving expression of phtD to phtK and a third operon that includes genes from phtM to phtV [10]. The function of argK, desI, amtA and phtU is known, while the function of the remaining genes remains uncertain [11–15]. The Pht cluster is also present in other phaseolotoxin-producing

pathovars, including P. syringae pv. actinidiae (a kiwi pathogen) and in a single strain of P. syringae pv. syringae CFBP3388, although in the latter the cluster organization is poorly conserved [16, 17]. Ribonucleotide reductase Different evidence has suggested that the Pht cluster was acquired in these pathovars by horizontal gene transfer, most likely from a Gram positive bacterium [18–20]. However, whether this cluster contains all the elements necessary for phaseolotoxin production is still unknown. Analysis of gene expression within the Pht cluster showed that most of the genes are transcribed at high levels at 18°C with a basal level of expression at 28°C, which agrees with the observed temperature-dependent pattern of phaseolotoxin synthesis, with the exception of phtL, which was expressed at both temperatures [10]. The mechanism by which P. syringae pv.