To construct the upp-based counterselection system for P. mendocina
NK-01 (Fig. 1), a 5-FU resistant strain was constructed initially. Based on
homologous recombination, the chromosomal upp gene was deleted.
The upstream and downstream homologous arm fragments of upp
gene (about 500 bp each) were amplified from the genomic DNA of P.
mendocina NK-01 using DNA polymerase with primers uppUP-F/
uppUP-R and uppDN-F/uppDN-R (Table 2). The up- and downstream
homologous arm fragments were then fused by overlapping PCR. The
overlapping PCR product of up- and downstream fragments was
digested with SacI/HindIII and subsequently ligated into pEX18Tc,
which was digested with the same enzymes, to produce recombinant
plasmid pEX18TcΔupp. The remaining ORF in the deletion plasmid encodes
12 N-terminal and 12 C-terminal amino acids of the intact Upp
protein. P. mendocina NK-01 was transconjugated with E. coli S17-1 harboring
pEX18TcΔupp. The plasmid cannot be autonomously replicated;
therefore, it has to integrate via single crossover homologous recombination
at the targeted chromosome. Positive clones should be TetR
(the plasmid pEX18TcΔupp confers Tet resistance) and CmR
. PCR was
used to analyze the chromosomal structure at the upp locus of the positive
clones, using primers uppOUT-F and uppOUT-R, designed according
to the genome sequence flanking the homologous arms, which
were used in both single- and double-crossover recombinants screen.
The verified single-crossover recombinant was cultured in LB without
antibiotics for more than 20 generations. The cultures were then plated
on LB agar plates containing 75 μg/ml of 5-FU and incubated at 30 °C for
18 h. PCR was used to analyze the chromosomal structure at the upp
locus of several 5-FUR colonies, using primers uppOUT-F and uppOUT-R
and the PCR amplicons were verified by sequencing