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Background  An available whole genome sequence for  Aspergillus flavus  provides the opportunity to characterize factors involved in pathogenicity and to elucidate the regulatory networks involved in aflatoxin biosynthesis. Functional analysis of genes within the genome is greatly facilitated by the ability to disrupt or mis-express target genes and then evaluate their result on the phenotype of the fungus. Large-scale functional analysis requires an efficient genetic transformation system and the ability to readily select transformants with altered expression, and usually requires generation of double (or multi) gene deletion strains or the use of prototrophic strains. However, dominant selectable markers, an efficient transformation system and an efficient screening system for transformants in  A. flavus  are absent.    Results  The efficiency of the genetic transformation system for  A. flavus  based on uracil auxotrophy was improved. In addition,  A. flavus  was shown to be sensitive to the antibiotic, phleomycin. Transformation of  A. flavus  with the  ble  gene for resistance to phleomycin resulted in stable transformants when selected on 100 μg/ml phleomycin. We also compared the phleomycin system with one based on complementation for uracil auxotrophy which was confirmed by uracil and 5-fluoroorotic acid selection and via transformation with the  pyr4  gene from  Neurospora crassa  and  pyrG  gene from  A. nidulans  in  A. flavus  NRRL 3357. A transformation protocol using  pyr4  as a selectable marker resulted in site specific disruption of a target gene. A rapid and convenient colony PCR method for screening genetically altered transformants was also developed in this study.    Conclusion  We employed phleomycin resistance as a new positive selectable marker for genetic transformation of  A. flavus . The experiments outlined herein constitute the first report of the use of the antibiotic phleomycin for transformation of  A. flavus . Further, we demonstrated that this transformation protocol could be used for directed gene disruption in  A. flavus . The significance of this is twofold. First, it allows strains to be transformed without having to generate an auxotrophic mutation, which is time consuming and may result in undesirable mutations. Second, this protocol allows for double gene knockouts when used in conjunction with existing strains with auxotrophic mutations.  To further facilitate functional analysis in this strain we developed a colony PCR-based method that is a rapid and convenient method for screening genetically altered transformants. This work will be of interest to those working on molecular biology of aflatoxin metabolism in  A. flavus , especially for functional analysis using gene deletion and gene expression.

Improved protocols for functional analysis in the pathogenic fungus Aspergillus flavus