A method for
retrofitting DNA in a single-copy or high-copy vector, such as a fosmid or BAC, whereby an artificial transposon is used to introduce a conditional multi-copy
origin of replication (“ori”) into the
DNA in said vector. Following random
in vitro or
in vivo transposition of the ori-containing transposon into
DNA in the single-copy or low-copy vector, the resulting
insertion clones are introduced into a special host strain that contains a
gene which encodes a polypeptide required for replication from the multi-copy ori. However, since the
gene for this polypeptide is expressed from a tightly-regulated inducible
promoter, the polypeptide is not expressed in the absence of
inducer. On addition of
inducer to the culture medium, the host
cell synthesizes the polypeptide, which in turn activates replication from the multi-copy ori, thereby increasing the amount of clone DNA synthesized by the
cell.