Prototrophy

Spontaneous mutations in bacteria are generally reversible. Consequently, in a medium where prototrophy confers no selective advantage, various auxotrophs (and other mutant types) should accumulate until for each type the number is such that the loss- by mutation to prototrophy should equal the gain by mutation to the corresponding auxotrophic condition.

The pyrG inserted into the gene replacement cassette was amplified from pCDA21 plasmid [40 ] and was used to generate a marker for prototrophy in the mutant strain.