A specific host factor binds at a cis-acting transcriptionally silent locus required for stability control of yeast plasmid pSR1

A cis-acting locus, Z, of plasmid pSRl functions in stable maintenance of the plasmid in the native host, Zygosaccharomyces rouxii. The Z locus was shown to be located in a 482 by sequence in the 5′ upstream region of an open reading frame, P, by subcloning various DNA fragments in a plasmid replicating via the ARS1 sequence of the Saccharomyces cerevisiae chromosome. Northern analysis revealed that the Z region is not transcribed in either the native host Z. rouxii or the heterologous host S. cerevisiae. The Z region is protected from microccocal nuclease attack in Z. rouxii but not in S. cerevisiae, its protection depending on the product of the S gene encoded by pSR1. Gel retardation assays suggested that a factor present in nuclear extracts of Z. rouxii cells, irrespective of the presence or absence of a resident pSRI plasmid, binds to a 111 by Rsal-Sacll sequence in the Z region. These findings suggest that a host protein binds to the Z locus and that the S product interacts with this DNA-protein complex and stabilizes pSRl.