The invention provides the dapE gene of Helicobacter pylori and H. pylori dapE.sup.- mutants and to methods of using the mutants to express foreign genes and immunize against foreign agents. The dapE gene can consist of the nucleotide sequence defined in SEQ ID NO:3. Nucleic acids of the gidA gene and ORF 2 of H. pylori are provided. Examples of these nucleic acids can be found in SEQ ID NO:1 and SEQ ID NO:5, respectively. Having provided these nucleic acids, hybridizing nucleic acids in accord with the description of hybridizing nucleic acids of dapE are also provided.