A putative cyclic peptide efflux pump encoded by the TOXA gene of the plant-pathogenic fungus Cochliobolus carbonum

Race 1 isolates of Cochliobolus carbonum are pathogenic on certain maize lines due to production of a host-selective cyclic tetrapeptide, HC-toxin. Flanking HTS1, which encodes the central enzyme in HC-toxin biosynthesis, a gene was identified and named TOXA. Like HTS1, TOXA occurred only in isolates of the fungus that make HC-toxin and was present as two linked copies in most toxin-producing isolates. HTS1 and TOXA were transcribed in the opposite orientation and their transcriptional start sites were 386 bp apart. The predicted product of TOXA was a 58 kDa hydrophobic protein with 10-13 membrane-spanning regions. The sequence was highly similar to several members of the major facilitator superfamily that confer resistance to tetracycline, methylenomycin, and other antibiotics. Although it was possible to mutate one copy or the other of TOXA by targeted gene disruption, numerous attempts to disrupt both copies in a single strain were unsuccessful, suggesting that TOXA is an essential gene in strains that synthesize HC-toxin. On the basis of its presence only in HC-toxin-producing strains, its proximity to HTS1 and its predicted amino acid sequence, we propose that TOXA encodes an HC-toxin efflux pump which contributes to self-protection against HC-toxin and/or the secretion of HC-toxin into the extracellular milieu.