Behaviour of a replicating mitochondrial DNA sequence fromAspergillus amstelodami inSaccharomyces cerevisiae andAspergillus nidulans

Summary An amplified sequence of mitochondrial DNA from a ragged (rgd) mutant ofAspergillus amstelodami has been shown to exist in multimeric circular form, suggesting that it is excised from the genome and can exist independently of it. This sequence has replicative (ARS) activity inSaccharomyces cerevisiae, and a subfragment responsible for this activity has been identified and sequenced. A homologous sequence fromAspergillus nidulans mtDNA also has ARS activity inS. cerevisiae. BothA. amstelodami andA. nidulans ARS elements have been incorporated into the integrative transformation vector pDJBI and the derived vectors used to transformA. nidulans. Inclusion of theA. nidulans ARS element enhanced the transformation frequency 5-fold relative to pDJBI. No increase in transformation frequency was evident with the ARS element fromA. amstelodami. The stability of transformants was variable but in comparison to pDJBI, ARS-containing plasmids were mitotically unstable inA. nidulans. Although plasmid DNAs could be rescued inEscherichia coli from undigested transformant DNA, no freely replicating plasmids were detected by Southern hybridisation.