Measure of Transformation Efficiency
Transformation effiency should be determined under conditions of cell excess. The standard plasmids used for determination of transformation efficiency in Escherichia coli are pBR322 or the smaller pUC series of vectors, different vectors however may be used to determine their transformation efficiency. The number of viable cells in a transformation reaction may range from 2x108 to 1011; most common methods of E. coli preparation yield around 1010 viable cells per reaction. 10-100 pg of DNA may be used for transformation, more DNA may be necessary for low-efficiency transformation. After transformation, plate separately 1% and 10% of the cells (dilute cells in media as necessary for ease of plating), but further dilution may be used for high efficiency transformation.
Transformation efficiency is measured in transformants or colony forming unit (cfu) per μg DNA used. A transformation efficiency of 1x108 cfu/μg for a small plasmid like pUC19 is roughly equivalent to 1 in 2000 molecules of the plasmid used being transformed. In E. coli, the theoretical limit of transformation efficiency for most commonly used plasmids would be over 1x1011 cfu/μg. In practice the best achievable result may be around 2-4x1010 cfu/μg for a small plasmid like pUC19, and considerably lower for large plasmids.
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