Background: Mangotoxin is an antimetabolite toxin that is produced by strains of Pseudomonas syringae pv.
syringae; 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 (mgoA) in mangotoxin production and virulence has been reported.
Results: 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 mgoA gene. Additionally, we
evaluated the importance of mgoC, mgoA and mgoD in mangotoxin production. A sequence analysis revealed an
operon-like organisation. A promoter sequence was located upstream of the mgoB gene and was found to drive lacZ
transcription. Two terminators were located downstream of the mgoD gene. RT-PCR experiments indicated that the
four genes (mgoBCAD) constitute a transcriptional unit. This operon is similar in genetic organisation to those in the
three other P. syringae pathovars for which complete genomes are available (P. syringae pv. syringae B728a, P. syringae
pv. tomato DC3000 and P. syringae pv. phaseolicola 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.
Conclusions: The results of this study confirm that mgoB, mgoC, mgoA and mgoD 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.
