Effect of fusion tags, E. coli strains and rare codon replacement on recombinant PiCV capsid protein expression. The three expression plasmids containing wild-type PiCV capsid gene were expressed in E. coli strains BL21 (DE3), BL21 (DE3)-pLysS and BL21 (DE3)-CodonPlus-RIPL. Furthermore, similar plasmids but containing the codon-optimized PiCV capsid gene were also expressed in BL21 (DE3) and BL21 (DE3)-pLysS. All examined recombinant PiCV capsid proteins were analyzed by SDS-PAGE and Western-blotting. Anti-6xHis tag monoclonal antibody was used to recognize the 6xHis-tagged (A) and Trx-tagged (C) Cap proteins. Anti-GST monoclonal antibody was used for detecting GST-tagged (B) Cap proteins. Lane M, pre-stained protein marker; lane 1, 3, 5, 7 and 9, before IPTG induction; lane 2, 4, 6, 8 and 10, after IPTG induction for 4 hrs cultivation. The solid triangle pinpoints the expressed Cap protein.