Construction of cloning cartridges for development of expression vectors in gram-negative bacteria

A cloning cartridge was constructed that can be inserted into a plasmid of choice to form an expression vector in which gene expression is inducible with an inexpensive inducer, sodium salicylate, at low concentrations. This cartridge consists of a 3.6-kb restriction fragment which contains the positive regulatory gene nahR from plasmid NAH7, a promoter, PG, that nahR regulates, a multiple cloning site, a transcription terminator, and a gene conferring tetracycline resistance. Within promoter PG of the cloning cartridge, a sequence of three nucleotides upstream of the ATG sequence encoding the initiation codon was altered to create an NdeI recognition site (CATATG) for cloning of the 5' end of a gene without affecting the distance between the transcription start site and the gene coding region. In addition, the 5' end of a gene can be converted into an NdeI recognition site without altering the amino acid sequence it encodes and then cloned into this cartridge for regulated expression. Several other synthetic restriction sites were also inserted downstream of the NdeI site for accepting the 3' end of a cloned gene. A derivative of this cloning cartridge lacking the NdeI sequence was also constructed for cloning and expression of a restriction fragment containing a gene(s) of unknown sequence. Use of the cloning cartridges in a broad-host-range plasmid has allowed successful cloning and inducible expression of several genes in all of the gram-negative bacterial tested to date. Protein production to at least 10% of the total soluble cell proteins was observed from a cloned gene expressed in Pseudomonas putida.