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Functional analysis of the yeast plasmid partition locus STB

Derivatives of the yeast 2μ plasmid with the cis‐acting locus STB (also called REP3) are stably maintained if two plasmid‐encoded proteins are present in trans. There are conflicting reports of both the extent of STB and its possible involvement in plasmid partition or copy number control. We have resolved the controversy by constructing 2µ derivatives with a conditional STB function, and showing that when STB is inactivated plasmids become concentrated in a small fraction of the population although the total number of plasmids remains unaltered. Moreover we show that STB consists of two functionally distinct domains which we call STB‐proximal and STB‐distal relative to the origin of replication. Although STB‐proximal is sufficient for proper partitioning, this function is severely disrupted by active transcription from neighbouring sequences. STB‐distal is important to protect STB‐proximal and ORI from such transcription, and can be effeciently replaced by a 94‐bp terminator fragment in an orientation‐dependent manner. We find that STB‐distal contains an additional element which depresses transcription from upstream promoters. We also describe the phenomenon of replicaton inhibition which we believe can exlain the anomalous instability of some yeast plasmids.