GENE-EXPRESSION FROM REPLICATING PLASMIDS IN ASPERGILLUS-NIDULANS

Plasmids bearing the AMA1 replicator from Aspergillus nidulans are cap able of extrachromosomal replication in this fungus as well as in othe r species. Synthetic plasmids bearing the moderately expressed argB ge ne and the highly expressed, inducible beta-galactosidase gene (bgaS) were introduced into fungal cells. Expression of both genes was monito red by Northern hybridization. It was demonstrated that transcription of bgaS is induced and repressed normally, irrespective of whether the gene is integrated into the chromosome or maintained on an extrachrom osomal supercoiled plasmid. Transcription of the strongly expressed bg aS gene stimulates transcription of the argB gene located on the same replicating plasmid irrespective of orientation. This effect also occu rs with chromosomally integrated vectors, but to a lesser extent. Epis omal vectors are present in 10 copies per nucleus, and the expression level of the argB gene is approximately proportional to copy number. H owever, the amount of mRNA transcribed from the highly expressed bgaS gene on the multi-copy replicating plasmid does not exceed that from s ingle-copy integrants. High levels of expression of the plasmid-borne gene do not affect plasmid mitotic stability or copy number.