my teacher gave me the following question:
YEp is usually transformed into cells already containing a 2μm plasmid. Why do recombinant YEps still function as multi-copy plasmids? Note that after transformation, the cell contains only 1 or 2 recombinant YEps and many endogenous 2μm plasmids, and that replication is only initiated once per cell cycle.
I know that 2μm plasmid can maintain a high copy number by switching to double rolling circle replication when its number/cell drops below a certain value. However, only few plasmids will replicate by RCR and the others will just replicate once.
My guess is that cells having multiple YEps have a selective advantage under laboratory conditions (i.e. are selected for using LEU+ as a marker on the YEp) and thus the ratio of cells containing multiple YEps increases. Is this why YEps based on 2μm plasmid are multi-copy plasmids?
However, I found an article by A.B. Futcher (Current Genetics 2, 201-205 1980) where he describes loss of endogenous 2μm plasmid in S.cerevisiae transformed with a chimeric plasmid. There, he states that even without selection for the recombinant plasmid, it will increase its copy number at the expense of the endogenous 2μm plasmid, eventually eliminating it. Why is that? I couldn't find an answer.