Submitted to: Fungal Genetics and Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 31, 2004
Publication Date: August 7, 2004
Citation: Chang, P.-K., Yu, J., Yu, J.-H. 2004. aflT, a MFS transporter-encoding gene located in the aflatoxin gene cluster, does not have a significant role in aflatoxin secretion. Fungal Genetics and Biology. 41:911-920. Interpretive Summary: Aflatoxin B1 is a secondary metabolite secreted by producing fungi, which can damage genetic materials of living cells if ingested. Most of the precursors prior to aflatoxin B1 formation are accumulated in mycelia. The difference of aflatoxin B1 from its precursors in cellular distribution implies that a transporting mechanism is operating. In this study, we disrupted the previously identified aflT transporter gene, present in the A. parasiticus aflatoxin gene cluster, and showed that aflT does not play a significant role in aflatoxin secretion. Transformants with the aflT gene deleted produced comparable amounts of aflatoxin B1, as well as B2, G1, and G2 as the wild-type. This result indicates that unidentified transporter(s) are able to compensate for AflT's function. Aflatoxin producing fungi thus may have redundant transporting systems for secreting/excluding toxin substances, such as fungicides. Future studies in identifying those transporting systems involved would help the management of toxic Aspergillus in the field, either by chemical or biological approaches. A reduction in aflatoxin contamination of crops would lead to increased profitability to the growers.
Technical Abstract: The aflT gene resides between the polyketide synthase gene pksA and the P450-encoding cypA gene in the aflatoxin gene cluster of Aspergillus parasiticus. It is a single copy gene in the genome of A. parasiticus SRRC 2043 and SU-1 and was also found at the same relative position in the genome of A. flavus isolates. The predicted AflT protein contained 14 transmembrane domains and had various degrees of amino acid identity (34 to 56 percent) to fungal transporters belonging to the major facilitator superfamily. Targeted deletion of aflT in A. parasiticus SU-1 yielded transformants that were morphologically similar to SU-1. These aflT-deleted mutants produced and secreted aflatoxins comparable to the parental strain. Real-time RT-PCR analysis showed that the expression of aflT was controlled neither by the aflatoxin pathway-specific activator AflR nor by the co-activator AflJ, which differed from the regulation of the aflatoxin biosynthetic genes pksA, nor1, ver1, and omtA. The FadA-dependent G-protein signaling pathway previously shown to govern aflatoxin biosynthesis and sporulation plays a role in the regulation of aflT expression.