Construction of pEGFP-3CLpro and pEGFP-C145A expression vectors. (A) Scheme of pEGFP-3CLpro and pEGFP-C145A construction. PCMV, human cytomegalovirus (CMV) immediate early promoter; EGFP, enhanced green fluorescent protein gene; MCS, multiple cloning sites; 3CLpro, SARS-CoV 3CLpro gene; C145A, 3CLpro C145A mutant gene; myc, myc-tag sequence; pA, SV40 early mRNA polyadenylation signal sequence. (B)The constructs were digested with restriction enzyme and separated on 1% agarose gel. M, DNA markers.