ROLE OF TYROSINE PHOSPHORYLATION-DEPHOSPHORYLATION IN COPY NUMBER CONTROL OF THE YEAST PLASMID-2 MICRON CIRCLE

A key feature of the copy control in the 2 micron circle plasmid of Sa ccharomyces cerevisiae is its ability to amplify when the copy number drops below the steady state value. The Flp protein encoded by the pla smid is an essential component of the amplification mechanism. A centr al regulatory event in amplification involves the phosphorylation/deph osphorylation of Tyr-343 of Flp. Tyrosine phosphorylation is achieved by a transesterification mechanism involving a specific phosphodiester within the 2 micron circle. The dephosphorylation is also a transeste rification reaction that uses a specific 5'-OH (generated during tyros ine phosphorylation) as the phosphoryl acceptor. A sum of four phospho rylation/dephosphorylation reactions, coordinated in sets of two, is t hought to invert the relative directions of a pair of replication fork s. This allows more than one copy of the plasmid to be made from a sin gle replication initiation event. In this paper we discuss the structu ral features of the Flp active site that control and coordinate the tr ansesterification reactions required for amplification.