PM-Toxin C

The Southern corn leaf blight (SCLB) epidemic of 1970 devastated fields of T-cytoplasm corn planted in monoculture throughout the eastern United States. The epidemic was driven by race T, a previously unseen race of Cochliobolus heterostrophus. A second fungus, Phyllosticta zeae-maydis, with the same biological specificity, appeared coincidentally. Race T produces T-toxin, while Phyllosticta zeae-maydis produces PM-toxin, both host-selective polyketide toxins necessary for supervirulence. The present abundance of genome sequences offers an opportunity to tackle the evolutionary origins of T- and PM- toxin biosynthetic genes, previously thought unique to these species. Using the C. heterostrophus genes as probes, we identified orthologs in six additional Dothideomycete and three Eurotiomycete species. In stark contrast to the genetically fragmented race T Tox1 locus that encodes these genes, all newly found Tox1-like genes in other species reside at a single collinear locus. This compact arrangement, phylogenetic analyses, comparisons of Tox1 protein tree topology to a species tree, and Tox1 gene characteristics suggest that the locus is ancient and that some species, including C. heterostrophus, gained Tox1 by horizontal gene transfer. C. heterostrophus and Phyllosticta zeae-maydis did not exchange Tox1 DNA at the time of the SCLB epidemic, but how they acquired Tox1 remains uncertain. The presence of additional genes in Tox1-like clusters of other species, although not in C. heterostrophus and Phyllosticta zeae-maydis, suggests that the metabolites produced differ from T- and PM-toxin.

We have tested directly the effect of Helminthosporium maydis T (Hmt) toxin and various analogs on the membrane potential formed in mitochondria isolated from a Texas (T) cytoplasmic male-sterile and a normal (N) corn. ATP, malate or succinate generated a membrane potential (negative inside) as monitored by the absorbance change of a cationic dye, safranine. The relative membrane potential (Deltapsi) could also be detected indirectly as (45)Ca(2+) uptake. Hmt toxin added to T mitochondria dissipated the steady state Deltapsi similar to addition of a protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP). Toxin analogs (Cpd XIII: C(41)H(68)O(12) and Cpd IV: C(25)H(44)O(6)), reduced native toxin (RT2C: C(41)H(84)O(13)) and Pm toxin (band A: C(33)H(60)O(8), produced by the fungus, Phyllosticta maydis) were effective in dissipating Deltapsi and decreasing Ca(2+) uptake with the following order: Pm (100) >> HmT (23-30) > Cpd XIII (11-25) >> RT2C (0-4-1.8) > Cpd IV (0.2-1.0). In contrast, the toxins and analogs had no effect on Deltapsi formed in N mitochondria. The striking similarities of the HmT toxin (band 1: C(41)H(68)O(13)) and Cpd XIII on T mitochondrial activities provide strong evidence supporting the correctness of the polyketol structure assigned to the native toxin. Since the Deltapsi in energized mitochondria is caused mainly by the electrogenic extrusion of H(+), the results support the idea that HmT toxin increases membrane permeability of T mitochondria to H(+). The host specificity of the toxin suggests that an interaction with unique target site(s) on the inner mitochondrial membrane of T corn causes H(+) leakage.