In many viruses and transposons, expression of some genes requires alternative reading of the genetic code, also called recoding. Such events depend on specific mRNA sequences and can lead to read through of an in-frame stop codon or to +1 or -1 frameshifting. Here, we addressed the issue of conservation of recoding rules between the yeast Saccharomyces cerevisiae and mammalian cells by establishing a versatile vector that can be used to study recoding in both species. We first assessed this vector by analysing the site of +1 frameshift of the Ty1 transposon. Two sequences from higher organisms were then tested in both yeast and mammalian cells: the gag-pol junction of human immunodeficiency virus type 1 (HIV-1) (a site of -1 frameshift), and the stop codon region of the replicase cistron from the tobacco mosaic virus (a site of UAG read through). We show that both sequences direct a high level of recoding in yeast. Furthermore, different mutations of the target sequences have similar effects on recoding in yeast and in mouse cells. Most notably, a strong decrease of frameshifting was observed in the absence of the HIV-1 stem-loop stimulatory signal. Taken together, these data suggest that mechanisms of some recoding events are conserved between lower and higher eukaryotes, thus allowing the use of S. cerevisiae as a model system to study recoding on target sequences from higher organisms.

A gene transfer system originally developed for Fusarium oxysporum has been applied to seven species of filamentous fungi of agricultural and industrial importance. This transformation system relies on the selection of mutants deficient in nitrate reductase by positive screening. Such mutants were recovered easily in all the fungi tested--without mutagenic treatments--through their resistance to chlorate. They were transformed by a plasmid vector (pAN301) carrying the Aspergillus nidulans wild-type gene (niaD). Transformation frequencies ranged from one to ten transformants/micrograms plasmid DNA. The general properties of the transformants were analyzed. Most of them are mitotically stable, and the integration of the vector into the host genome frequently occurred in a tandem fashion.