This has allowed the development of a balanced - lethal system for the expression of heterologous antigens in vaccine strains using vectors containing the wild - type asd gene from s . typhimurium and asd mutant salmonella hosts Sds一page和westem一blot检测表明，菌苗侯选株能够分别表达出相对分子质量为64kda的尿素酶b蛋白与77kda的尿素酶b和热休克蛋白a的融合蛋白。
