Characterisation of the mgo operon in Pseudomonas syringae pv. syringae UMAF0158 that is required for mangotoxin production

Abstract

<p>Abstract</p> <p>Background</p> <p>Mangotoxin is an antimetabolite toxin that is produced by strains of <it>Pseudomonas syringae </it>pv. <it>syringae</it>; mangotoxin-producing strains are primarily isolated from mango tissues with symptoms of bacterial apical necrosis. The toxin is an oligopeptide that inhibits ornithine N-acetyl transferase (OAT), a key enzyme in the biosynthetic pathway of the essential amino acids ornithine and arginine. The involvement of a putative nonribosomal peptide synthetase gene (<it>mgo</it>A) in mangotoxin production and virulence has been reported.</p> <p>Results</p> <p>In the present study, we performed a RT-PCR analysis, insertional inactivation mutagenesis, a promoter expression analysis and terminator localisation to study the gene cluster containing the <it>mgo</it>A gene. Additionally, we evaluated the importance of <it>mgo</it>C, <it>mgo</it>A and <it>mgo</it>D in mangotoxin production. A sequence analysis revealed an operon-like organisation. A promoter sequence was located upstream of the <it>mgo</it>B gene and was found to drive <it>lac</it>Z transcription. Two terminators were located downstream of the <it>mgo</it>D gene. RT-PCR experiments indicated that the four genes (<it>mgo</it>BCAD) constitute a transcriptional unit. This operon is similar in genetic organisation to those in the three other <it>P. syringae </it>pathovars for which complete genomes are available (<it>P. syringae </it>pv. <it>syringae </it>B728a, <it>P. syringae </it>pv. <it>tomato </it>DC3000 and <it>P. syringae </it>pv. <it>phaseolicola </it>1448A). Interestingly, none of these three reference strains is capable of producing mangotoxin. Additionally, extract complementation resulted in a recovery of mangotoxin production when the defective mutant was complemented with wild-type extracts.</p> <p>Conclusions</p> <p>The results of this study confirm that <it>mgo</it>B, <it>mgo</it>C, <it>mgo</it>A and <it>mgo</it>D function as a transcriptional unit and operon. While this operon is composed of four genes, only the last three are directly involved in mangotoxin production.</p