Enhanced expression of PCV2 capsid protein in Escherichia coli and Lactococcus lactis by codon optimization

Capsid protein (Cap) of porcine circovirus type 2 (PCV2) encoded by orf2 is a main structural protein with strong immunoreactivity. However, capsid protein is expressed poorly in prokaryotic organisms because of differences in codon usage. In this study, we introduce 24 synonymous mutations into orf2 by mutagenic primers and overlap extension polymerase chain reaction (OE-PCR) technique. Fourteen rare codons of orf2 were replaced with preferable codons used in Escherichia coli cells. Moreover, the nuclear localization signal (NLS) region rich in rare codon clusters at the 5' end was deleted. The codon-optimized genes demonstrated higher levels of expression compared with wild-type genes. The influence of rare codons on the gene expression was eliminated by mutation. Western blot analysis confirmed the immunoreactivity of the proteins expressed by mutated genes. Further testing demonstrated that the mutated genes were also expressed successfully in Lactococcus lactis NZ9000. The immunologically active Cap proteins produced by recombinant strains have the potential applications for serological diagnostic assays and vaccine development against PCV2-associated diseases.