ARS | Publication request: Functional and phylogenetic analysis of the Aspergillus ochraceoroseus aflQ (ordA) gene ortholog

Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 24, 2012
Publication Date: April 24, 2012
Citation: Cary, J.W., Harris Coward, P.Y., Ehrlich, K., Moore, G.G., Wei, Q., Bhatnagar, D. 2012. Functional and phylogenetic analysis of the Aspergillus ochraceoroseus aflQ (ordA) gene ortholog. Mycologia. 104(4)857-864.

Interpretive Summary: In many fungi, the genes responsible for producing toxic compounds such as aflatoxins (AFs) are often clustered together on the chromosome. We have identified the aflatoxin gene cluster in the fungus Aspergillus ochraceoroseus (Ao). The genes and their organization in the Ao cluster are more similar to those found the fungus Aspergillus nidulans (An) that makes the aflatoxin precursor sterigmatocystin (ST) than they are to the genes found in other AF-producing Aspergillus species like A. flavus. Unlike An, Ao makes AF though the Ao AF gene cluster does not contain two of the genes, aflQ and aflP, eeded for the conversion of ST to AF. In this study we cloned and characterized the Ao aflQ gene. The information obtained in this study indicates that Ao may be ancestral to both AF and ST producers found today. Retainment or loss of AF production may have been driven by the fungus’ need to adapt to their specific environmental niche.

Technical Abstract: Within the Aspergillus parasiticus and A. flavus aflatoxin (AF) biosynthetic gene cluster the aflQ (ordA) and aflP (omtA) genes encode an oxidoreductase and methyltransferase, respectively. These genes are required for the final steps in the conversion of sterigmatocystin (ST) to aflatoxin B1 (AFB1). Aspergillus nidulans harbors a gene cluster that produces ST, as the aflQ and aflP orthologs are either non-functional or absent in the genome. Aspergillus ochraceoroseus produces both AF and ST and it harbors an AF/ST biosynthetic gene cluster that is organized much like the A. nidulans ST cluster. The A. ochraceoroseus cluster also does not contain aflQ or aflP orthologs. However the ability of A. ochraceoroseus to produce AF would indicate that functional aflQ and aflP orthologs are present within the genome. Utilizing degenerate primers based on conserved regions of the A. flavus aflQ gene and an A. nidulans gene demonstrating the highest level of homology to aflQ, a putative aflQ ortholog was PCR amplified from A. ochraceoroseus genomic DNA. The A. ochraceoroseus aflQ ortholog demonstrated 57% amino acid identity to A. flavus AflQ. Transformation of an O-methylsterigmatocystin (OMST)-accumulating A. parasiticus aflQ mutant with the putative A. ochraceoroseus aflQ gene restored AF production. Though the aflQ gene does not reside in the AF/ST cluster it appears to be regulated in a manner similar to other A. ochraceoroseus AF/ST cluster genes. Phylogenetic analysis of AflQ and AflQ-like proteins from a number of ST- and AF-producing Aspergilli indicates that A. ochraceoroseus may be ancestral to A. nidulans and A. flavus.