Yeast Vectors, Yeast Episomal Plasmid YEP, 2 Mu M Plasmid, Beggs, Histidine Biosynthesis, REP2 Genes, REP1 Genes, REP3 Genes

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	Yeast Vectors - Yeast Episomal Plasmid - YEP - These are naturally available plasmids of size 6.3 kb. This plasmid is also called as 2 μm plasmid. YEP contains an origin of replication and three genes, REP3 (cis-acting elements) REP2 and REPI. 50-100 copies of plasmids exist per cell, which represent 2-4% of total yeast genome.
About 50% of YEP is essential for replication and maintenance and the remaining is dispensable. These plasmids have no markers and both the halves are useful. Hence Beggs in 1978 constructed a shuttle vector by using the essential portion of 2 μM plasmid and pBR 322. The marker gene encodes the enzyme required for histidine biosynthesis. Hence the recombinant transformants were selected by growing the yeast mutant for histidine biosynthesis on a medium lacking histidine.