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In a previous study, a 24-kilodalton (kDa) protein surface antigen of Legionella pneumophila was cloned into Escherichia coli and found to be expressed on the host cell surface. Subsequently, a site-directed mutation in this gene (designated mip) in L. pneumophila was found to impair the capacity of this bacterium to initiate intracellular infection in human macrophages. The work presented here indicates that the antigenic gene product is distinct from the 24- to 29-kDa major outer membrane protein of L. pneumophila. In addition, the antigen was identified as a highly basic protein on two-dimensional nonequilibrium polyacrylamide gels and on two-dimensional monoclonal antibody immunoblots. When the DNA fragment encoding this protein was sequenced, a long open reading frame of 699 base pairs was identified within a region to which antigen expression was previously mapped. mip mRNA isolated from both L. pneumophila and transformed E. coli had the same 5' end, as determined by primer extension analysis, indicating that the same promoter sequences are used in both species. A likely factor-independent transcriptional terminator was found 20 residues downstream of the stop codon, suggesting that mip is encoded on a monocistronic message. The inferred polypeptide began with a possible 20- to 24-residue signal sequence, and, as predicted by two-dimensional electrophoresis, had a molecular weight of 24,868 and was a potent polycation with an estimated pI of 9.8.
Engleberg et al. (Sat,) studied this question.