Cloning and maintenance of the housefly sodium channel gene using low copy number vector and two sequential host strains

In spite of the long history of recombinant DNA technology, some genes have not been successfully cloned in Escherichia coli. This is probably due to the toxic effects of the expressed foreign gene product on E. coli. In initial attempts to clone the full-length Vssc1 voltage-sensitive sodium channel α-subunit gene from houseflies, we used one of the most popular vectors and hosts but were unable to retrieve any intact clone. By using two vectors with different copy numbers and two alternate E. coli host strains, we found that the combined use of a low copy number vector (pALTER-1) and an E. coli host strain that suppresses plasmid replication (ABLE-K) is essential to obtain intact full-length Vssc1 clone. However, since the ABLE-K strain was not a suitable host for the long-term maintenance of Vssc1 gene due to its recombination-positive genotype, it was necessary to transfer the Vssc1 plasmid from the primary host to a secondary host with a recombination-minus genotype (Stbl2) to minimize the chances of deletion or rearrangement. We believe that this cloning strategy, with a low copy number vector and the sequential use of two E. coli strains, will be also applicable for the cloning of other toxic genes.