ABSTRACT
BACKGROUND: Plasmid-less, engineered Bacillus strains have several advantages over plasmid-carrier variants. Specifically, their stability and potential ecological safety make them of use in industrial applications. As a rule, however, it is necessary to incorporate many copies of a key gene into a chromosome to achieve strain performance that is comparable to that of cells carrying multiple copies of a recombinant plasmid. RESULTS: A plasmid-less B. subtilis JE852-based strain secreting glutamyl-specific protease (GSP-the protein product of the mpr gene from B. amyloliquefaciens) was constructed that exhibits decreased levels of other extracellular proteases. Ten copies of an mprB.amy cassette in which the GSP gene was placed between the promoter of the B. amyloliquefaciens rplU-rpmA genes and the Rho-independent transcription terminator were ectopically inserted into designated (3 copies) and random (7 copies) points in the recipient chromosome. The resulting strain produced approximately 0.5 g/L of secreted GSP after bacterial cultivation in flasks with starch-containing media, and its performance was comparable to an analogous strain in which the mprB.amy cassette was carried on a multi-copy plasmid. CONCLUSION: A novel strategy for ectopically integrating a cassette into multiple random locations in the B. subtilis chromosome was developed. This new method is based on the construction of DNA fragments in which the desired gene, marked by antibiotic resistance, is sandwiched between "front" and "back" portions of random chromosomal DNA restriction fragments. These fragments were subsequently inserted into the targeted sites of the chromosome using double-cross recombination. The construction of a marker-free strain was achieved by gene conversion between the integrated marked gene and a marker-less variant carried by plasmid DNA, which was later removed from the cells.
