Cloning and characterization of the asd gene of Salmonella typhimurium: use in stable maintenance of recombinant plasmids in Salmonella vaccine strains

The asd mutants of Salmonella typhimurium have an obligate requirement for diaminopimelic acid (DAP) and will undergo lysis in environments deprived of DAP. 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 Streptococcus mutans and asd mutant Salmonella hosts [Nakayama et al., Biotechnology 6 (1988) 693-697]. We have cloned the asd gene from S. typhimurium, characterized the gene product and used this gene to construct Asd⁺ expression cloning vectors. In addition we have constructed an asd cassette and a transposon derived from Tn5 that allow the rapid modification of other vectors for use with Δasd vaccine strains of S. typhimurium adding versatility to the Asd⁺ vector/Δasd host system of plasmid maintenance.