Abstract
Pseudomonas syringae pv. phaseolicola produces phaseolotoxin in a temperature dependent manner, being optimally synthesized between 18°C and 20°C, while no detectable amounts are present above 28°C. The Pht cluster, involved in the biosynthesis of phaseolotoxin, contains 23 genes that are organized in five transcriptional units. The function of most of the genes from the Pht cluster is still unknown and little information about the regulatory circuitry leading to expression of these genes has been reported. The purpose of the present study was to investigate the participation of pht genes in the regulation of the operons coded into the Pht cluster. We conducted Northern blot, uidA fusions and reverse transcription-PCR assays of pht genes in several mutants unable to produce phaseolotoxin. This allowed us to determine that, in P. syringae pv. phaseolicola NPS3121, genes phtABC are essential to prevent their own expression at 28°C, a temperature at which no detectable amounts of the toxin are present. We obtained evidence that the phtABC genes also participate in the regulation of the phtD, phtM and phtL operons. According to our results, we propose that PhtABC and other Pht product activities could be involved in the synthesis of the sulfodiaminophosphinyl moiety of phaseolotoxin, which indirectly could be involved in the transcriptional regulation of the phtA operon.
Highlights
Some strains of Pseudomonas syringae pv. phaseolicola and P. syringae pv. actinidiae, as well as P. syringae pv. syringae strain CFBP3388, produce phaseolotoxin, an unspecific toxin that inhibits the biosynthesis of arginine and polyamines [1,2,3]
There are several gene products coded outside the Pht cluster involved in phaseolotoxin regulation: the GacA/GacS system, which is necessary for expression of operons within the Pht cluster at 18 ̊C; the global regulator IHF, participating in the repression control of the phtD operon, and, an unknown repressor protein able to mediate an efficient argK repression [25, 28, 29]
Other genes coded into the Pht cluster participate: the gene product of phtL is involved in the regulation of the phtM operon [18, 38], whereas the products of genes phtABC participate in the transcriptional repression of gene argK at temperatures not permissive for phaseolotoxin biosynthesis [29]
Summary
Some strains of Pseudomonas syringae pv. phaseolicola and P. syringae pv. actinidiae, as well as P. syringae pv. syringae strain CFBP3388, produce phaseolotoxin, an unspecific toxin that inhibits the biosynthesis of arginine and polyamines [1,2,3]. Actinidiae, as well as P. syringae pv. Syringae strain CFBP3388, produce phaseolotoxin, an unspecific toxin that inhibits the biosynthesis of arginine and polyamines [1,2,3]. Phaseolotoxin biosynthesis depends on phtABC genes expression es), co-financed by the Fondo Europeo de Desarrollo Regional (FEDER) to JM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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