Aspergillus nidulans is one of the model ascomycete fungi. Transposition events have never been described in this organism. We have determined that this organism has at least 13 copies of a Fot1-related element. These copies are transcribed, non-methylated and polymorphic in various wild isolates. In spite of this, we have failed to isolate transposon insertions when the resident niaD gene is used as a transposon trap. This contrasts with the situation described previously in Fusarium oxysporum. We show that two elements of F. oxysporum, Fot1 and impala, transpose efficiently in A. nidulans. We have developed the impala system by tagging it with the yA gene. This permits the visual detection of the transposon by the colour of the conidiospores. We demonstrate that no endogenous transposase of A. nidulans is able to act in trans on a defective impala element, whereas its own transposase driven by two different promoters is able to mobilize this element. The frequency of excision of these modified elements is between 10(-4) and 10(-5). Loss of the transposable element occurs in about 10% of all excision events. In the remaining 90%, the transposon seems to be integrated at random positions in the genome. The availability of mitochondrially inherited mutations has allowed us to demonstrate that hybrid dysgenesis is apparently absent in A. nidulans. The development of this system opens the way to investigating the mechanism underlying the paucity of transposition events leading to visible phenotypes. It should allow us to develop efficient gene-tagging tools, useful in this and other fungi.

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Inspired by the successful work of converting Saccharomyces cerevisiae into an microorganism capable of synthesizing hydrocortisone, a 27-carbon molecule, from ethanol, a 2-carbon molecule, this review provides an overview of the potential of yeast as a recombinant organism in the 21st century. Yeast has been used by man for more than 6,000 years, and is still paving the way to new discoveries. It was the first eukaryotic organism to be sequenced, in 1996, and the first to produce hydrocortisone in 2003. In addition, extensive genome-wide analyses have been performed with yeast. In this review, we discuss the pros and cons of using yeast to produce small therapeutic molecules. It is obvious that S. cerevisiae has a cutting edge advantage of being a well-known organism and time will tell if yeast "biohydrocortisone" is a unique example or the beginning of a long list of yeast bioproducts. Other organisms, such as plants and bacteria, are competing with yeast. Bacteria produce a wealth of marketed molecules and plants are capable of producing extremely complex molecules with an unbeatable yield. However, S. cerevisiae offers a unique mix of the simplicity of a recombinant organism combined with the complexity of a eukaryote.

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