Abstract
Seamless modification of bacteria chromosome is widely performed both in theoretical and in practical research, such as functional genome analysis and metabolic engineering. For this purpose, excellent counter-selection marker genes with high selection stringency are urgent needed. Lysis gene E from bacteriophage PhiX174, which is of universal and powerful killing effect on Gram-negative strains, was developed and optimized as a generic counter-selection marker in this paper.
Lysis gene E was firstly constructed under the control ofpL promoter. At high temperature such as 42 °C, inducible expression of Lysis gene E could effectively promote the death ofEscherichia coli MG1655. Seamless modification using E as a counter-selection marker also successfully conducted with high ration of positive recombinants. It also works in another Gram-negative strainSerratia marcescens under the control of Arac/PBAD regulatory system. Through combining lysis gene E and kil , the selection stringency frequency ofpL -kil-sd-E cassette in E. coli arrived at 4.9×10−8 and 3.2×10−8 at two test loci, which is very close to the best counter-selection system, inducible toxins system. Under the control of Arac/PBAD,selection stringency of PBAD-kil-sd-E in S. marcescens arrived mostly at the level of 10−7 at four test loci. By introducing araC gene harboring plasmid pKDsg-ack, 5- to 18- fold improvement of selection stringency was observed at all these loci, and a surprising low selection stringency frequency 4.9×10−9 was obtained at marR-1locus. This is the lowest selection stringency frequency for counter-selection reported so far. Similarly, at araB locus of E. coli selection stringency frequency of PBAD-kil-sd-E was improved 170- fold to 3×10−9 after introducing plasmid pKDsg-ack.
In conclusion, we have developed and optimized a newly universal counter-selection marker based on lysis gene E . The best selection stringency of this new marker exceeds the inducible toxins system several fold. We provide a valuable counter-selection marker with high selection stringency to the genome editing toolbox.
Keyword: Counter-selection, Selection stringency frequency, lysis gene E , Red recombination, metabolic engineering