synthase that catalyzes the synthesis of a linear polyketide that forms the backbone
structure of fumonisins. Additionally, the FUM8 gene runs the condensation of the
linear polyketide with alanine to produce fumonisins [9], and FUM21 encodes a Zn
(II)2Cys6 DNA-binding transcription factor that positively regulates FUM expression [108]. The cluster also encodes an ABC transporter (FUM19) that provides
a sort of self-protection by exporting the toxin from the cell and reducing its cellular
concentration. The number, order, and orientation of genes within FUM cluster were
specified to be similar for closely related F. verticillioides and F. proliferatum but
also for F. oxysporum; however, only one fumonisin-producing strain O-1890 has
been described in detail [6, 9, 103]. Nevertheless, the sequences flanking the FUM
cluster seem to alter in F. verticillioides, F. proliferatum, and F. oxysporum, showing
different genomic contexts of the FUM cluster in these three species and, possibly,
also in other producers, like F. nygamai [7]. Proctor et al. (2003) determined the
genomic context of the FUM cluster by the sequence analysis of the DNA regions
flanking each side of the cluster. The analysis shows five different genomic context
or genetic environments (GE), namely, GC1, GC2, GC3a, GC3b, and GC4. The one
designed GC1 is devoted to the full FUM cluster in F. verticillioides [103] and for
FUM cluster remnant in F. musae [109], where ORF20 and ORF21 represent
pseudogenes, most likely the homologs of the F. graminearum gene
FGSG_00274, and are flanking the FUM21 side, whereas ZBD1 and ZNF1 are
flanking the FUM19 side. The GC2 was detected in all African clade species
examined, where ANK1 and GAT1 are flanking the FUM19 side and ZBD1 and
MFS1 are flanking the FUM21 side. The GC3a and GC3b were shown in Americanclade species F. anthophilum and F. bulbicola, respectively. They have a similar
structure with three genes (CPM1, MF2, and DOX1) flanking the FUM19 side,
differing in the FUM21-flanking region: in the GC3a, FUM21 is flanked by CPM2
and TSP1, while in the GC3b, there was no evidence for these genes. The GC4 was
observed in F. oxysporum (FRC O-1890 strain), where there was no full-length gene
within the Σ2800 bp region upstream of FUM21 and a homolog of CPM1 was
flanking the FUM19 side [59].
The fumonisin biosynthesis starts when the FUM1 product catalyzes the condensation of nine acetate and two methyl units to form a linear, 18-carbon-long
polyketide. The polyketide should be identical or similar in structure to 10,14dimethyl octadecanoic acid. However, it is possible that the polyketide does not
exist as a free acid but remains covalently attached to the phosphopantetheinyl
cofactor of the PKS instead [110]. In the second step, the FUM8-encoded protein
Fum8p catalyzes the condensation of the linear polyketide and alanine to yield
a linear molecule that is 20 carbons long and has an amine at C-2, a carbonyl at
C-3, and methyl residues at the C-12 and C-16 [9, 111, 112]. A third step of the
pathway is catalyzed by the FUM6-encoded Fum6p protein and consists of the
hydroxylation of the polyketide-amino acid condensation product at the C-14 and
C-15 [9, 113]. The fourth, fifth, and sixth steps are the following reactions: C-3
carbonyl reduction, C-10 hydroxylation, and C-14/C-15 esterification, respectively.
Metabolic profiling of numerous F. verticillioides mutants indicated that each of
these reactions can occur independently from the others. The C-3 carbonyl reduction
222
Ł. Stępień et al.
structure of fumonisins. Additionally, the FUM8 gene runs the condensation of the
linear polyketide with alanine to produce fumonisins [9], and FUM21 encodes a Zn
(II)2Cys6 DNA-binding transcription factor that positively regulates FUM expression [108]. The cluster also encodes an ABC transporter (FUM19) that provides
a sort of self-protection by exporting the toxin from the cell and reducing its cellular
concentration. The number, order, and orientation of genes within FUM cluster were
specified to be similar for closely related F. verticillioides and F. proliferatum but
also for F. oxysporum; however, only one fumonisin-producing strain O-1890 has
been described in detail [6, 9, 103]. Nevertheless, the sequences flanking the FUM
cluster seem to alter in F. verticillioides, F. proliferatum, and F. oxysporum, showing
different genomic contexts of the FUM cluster in these three species and, possibly,
also in other producers, like F. nygamai [7]. Proctor et al. (2003) determined the
genomic context of the FUM cluster by the sequence analysis of the DNA regions
flanking each side of the cluster. The analysis shows five different genomic context
or genetic environments (GE), namely, GC1, GC2, GC3a, GC3b, and GC4. The one
designed GC1 is devoted to the full FUM cluster in F. verticillioides [103] and for
FUM cluster remnant in F. musae [109], where ORF20 and ORF21 represent
pseudogenes, most likely the homologs of the F. graminearum gene
FGSG_00274, and are flanking the FUM21 side, whereas ZBD1 and ZNF1 are
flanking the FUM19 side. The GC2 was detected in all African clade species
examined, where ANK1 and GAT1 are flanking the FUM19 side and ZBD1 and
MFS1 are flanking the FUM21 side. The GC3a and GC3b were shown in Americanclade species F. anthophilum and F. bulbicola, respectively. They have a similar
structure with three genes (CPM1, MF2, and DOX1) flanking the FUM19 side,
differing in the FUM21-flanking region: in the GC3a, FUM21 is flanked by CPM2
and TSP1, while in the GC3b, there was no evidence for these genes. The GC4 was
observed in F. oxysporum (FRC O-1890 strain), where there was no full-length gene
within the Σ2800 bp region upstream of FUM21 and a homolog of CPM1 was
flanking the FUM19 side [59].
The fumonisin biosynthesis starts when the FUM1 product catalyzes the condensation of nine acetate and two methyl units to form a linear, 18-carbon-long
polyketide. The polyketide should be identical or similar in structure to 10,14dimethyl octadecanoic acid. However, it is possible that the polyketide does not
exist as a free acid but remains covalently attached to the phosphopantetheinyl
cofactor of the PKS instead [110]. In the second step, the FUM8-encoded protein
Fum8p catalyzes the condensation of the linear polyketide and alanine to yield
a linear molecule that is 20 carbons long and has an amine at C-2, a carbonyl at
C-3, and methyl residues at the C-12 and C-16 [9, 111, 112]. A third step of the
pathway is catalyzed by the FUM6-encoded Fum6p protein and consists of the
hydroxylation of the polyketide-amino acid condensation product at the C-14 and
C-15 [9, 113]. The fourth, fifth, and sixth steps are the following reactions: C-3
carbonyl reduction, C-10 hydroxylation, and C-14/C-15 esterification, respectively.
Metabolic profiling of numerous F. verticillioides mutants indicated that each of
these reactions can occur independently from the others. The C-3 carbonyl reduction
222
Ł. Stępień et al.