Cloning and characterization of the Neisseria meningitidis asd gene

The asd mutants of Gram^- and some Gram⁺ bacteria have an obligate requirement for diaminopimelic acid (DAP), an essential constituent of the cell wall of these organisms. In environments deprived of DAP, i.e., mammalian tissues, they will undergo lysis. This has previously been exploited to develop vaccine strains of Salmonella typhimurium and Streptococcus mutaons. As a first step for the development of a biosafe Neisseria meningitidis laboratory strain, we have cloned the asd from wild-type strain B16B6 by complementation of an Eschirichia coli asd mutant. By subcloning and insertion mutagenesis, the N. meningitidis asd was localized to a 1.5-kb DNA fragment. In a T7 RNA polymerase-T7 promoter expression system, a 38-kDa protein was strongly expressed from this DNA fragment. The N-terminal amino acid (aa) sequence was deduced from the nucleotide sequence, which was determined with the help of an in-frame Asd'::'LacZ protein fusion. A comparison of the N-terminal aa of the Asd proteins from N. meningitidis and E. coli revealed 70% identity, suggesting that the Asd protein may be highly conserved among Gram^- bacteria.