We have developed a DNA vaccine encoding a fusion protein of ubiquitin (Ub) and target proteins at the N-terminus for effective induction of antigen-specific CD8(+) T cells. A series of expression plasmids encoding a model antigen, ovalbumin (OVA), fused with mutated Ub. was constructed. Western blotting analyses using COS7 cells transfected with these plasmids revealed that there were three types of amino acid causing different bindin capacities between Ub and OVA. Natural Ub with a C-terminal glycine readily dissociated from OVA; on the other hand, artificially mutated Ub, the C-terminal amino acid of which had been exchanged to valine or arginine, stably united with the polypeptide, while Ub with a C-terminal alanine partially dissociated. The ability of DNA vaccination to induce OVA-specific CD8(+) T cells closely correlated with the stability of Ub fusion to OVA. Our strategy could be used to optimize the effect of genetic vaccines on the induction of CD8(+) T cells. (c) 2007 Elsevier Inc. All rights reserved.