The silent P mating type locus in fission yeast contains two autonomously replicating sequences

We show that in fission yeast two DNA fragments at the silent P mating type locus provide plasmids with the capability of autonomous replication. Bacterial vectors containing these sequences replicate In a polymeric form in fission yeast very much like plasmids with the commonly used replication sequence ars1, do. There are, however, several differences between the two new ars sequences. The percentage of cells containing the plasmld during selection, the plasmld copy number and the plasmid segregation during mitosis are all dependent on the choice of the are sequence. A DNA fragment with ars activity from the left side of the silent P cassette represses the expression of the marker gene, ura4+, at least three hundred fold compared to plasmids containing only the other new ars sequence or only ars1. The Importance of replication in this promoter independent transcriptlonal regulation Is further substantiated by the fact that the repression Is partially released In the presence of ars7 on the same plasmld