Signals for transcription initiation and termination in the Saccharomyces cerevisiae plasmid 2 micron circle

By S1 nuclease protection experiments and primer extension analysis, we determined precisely the cap and polyadenylation sites of transcripts from the four genes of the yeast 2 micron circle plasmid, as well as those of other plasmid transcripts of unknown function. In addition, we used deletion analysis to identify sequences necessary for polyadenylation in plasmid transcripts. Our results indicate that plasmid genes constitute independent transcription units and that plasmid mRNAs are not derived by extensive processing of precursor transcripts. In addition, we found that the D coding region of 2 micron circle is precisely encompassed by a polyadenylated transcript, suggesting that this coding region constitutes a functional plasmid gene. Our identification of the position of plasmid polyadenylation sites and of sequences necessary for polyadenylation provides support for a tripartite signal for polyadenylation as proposed by Zaret and Sherman (K.S. Zaret and F. Sherman, Cell 28:563-573, 1982). Finally, these data highlight salient features of the transcriptional regulatory circuitry that underlies the control of plasmid maintenance in the cell.