Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 8, 2009
Publication Date: June 3, 2009
Citation: Cary, J.W., Ehrlich, K., Beltz, S.B., Harris Coward, P.Y., Klich, M.A. 2009. Characterization of the Aspergillus ochraceoroseus aflatoxin/sterigmatocystin biosynthetic gene cluster. Mycologia. 101(3):352-362. Interpretive Summary: Filamentous fungi produce a number of secondary metabolic compounds that have been shown to be both of great value (i.e. antibiotics and anti-hypercholesterolemics) and great harm (i.e. aflatoxins and trichothecenes). The genes responsible for producing the secondary metabolite are often clustered together on the chromosome. The genes required for the production of the toxic compound aflatoxin (AF) are present in fungi as a gene cluster. We have identified the aflatoxin gene cluster in the fungus, Aspergillus ochraceoroseus (Ao). Interestingly, the genes present in the cluster are more similar to those found in the fungus Aspergillus nidulans (An) that makes the aflatoxin precursor sterigmatocystin (ST), than they are to the genes found in other aflatoxin-producing Aspergillus species like A. flavus. However, the organization of the Ao aflatoxin cluster is unlike that of An in that there has been a significant rearrangement of the genes in the cluster. The information obtained in this study indicates that the ancestral parent of both AF and ST producers may have been an AF-producer and at some point during evolution the AF producers diverged with one branch continuing to make AF while the other branch lost the ability to produce AF. Retainment or loss of AF production may have been driven by the fungus’ need to adapt to their specific environmental niche.
Technical Abstract: Production of the carcinogenic aflatoxins has been reported from members of Aspergillus section Flavi, Aspergillus section Nidulantes, and a newly proposed section, Aspergillus section Ochraceorosei that consists of Aspergillus ochraceoroseus and A. rambellii. Unlike members of section Flavi, A. ochraceoroseus and A. rambellii have been shown to accumulate both aflatoxin (AF) and the aflatoxin precursor sterigmatocystin (ST). Though morphologically distinct from A. nidulans, molecular characterization of A. ochraceoroseus AF/ST genes and physiological characteristics of AF/ST production indicated that A. ochraceoroseus is more closely related to A. nidulans than to A. flavus. The A. nidulans ST gene cluster is organized differently from the A. flavus AF gene cluster; we therefore determined the genetic organization of the AF/ST biosynthetic cluster in A. ochraceoroseus. Sequencing of overlapping lambda clones and genomic PCR fragments obtained by gene-walking techniques demonstrated that the A. ochraceoroseus AF/ST gene cluster is organized much like the A. nidulans ST gene cluster, except that the region from aflN to aflW is located directly upstream of aflC and in reverse orientation such that aflW represents the distal end and aflY the proximal end of the cluster. The A. ochraceoroseus cluster genes demonstrated 62-76% nucleotide identity to their A. nidulans ST cluster gene homologs. Transformation of an A. nidulans aflR mutant with the A. ochraceoroseus aflR restored ST production in A. nidulans transformants. PCR amplification of A. rambellii genomic DNA demonstrated that the AF/ST gene cluster is organized in the same manner as that of A. ochraceoroseus.