Swapping genes

Abstract

Haemophilus and Neisseria spp. are common, normal inhabitants of the human upper respiratory tract but can occasionally cause life-threatening infections. Many species of both of these genera are naturally competent for DNA transformation, a capacity that allows them to take up fragments of DNA from the environment and incorporate this DNA into their chromosomes. The sequence required for uptake has been defined for Haemophilus influenzae (HmUS) as a 29-bp consensus sequence with a conserved 9-bp core sequence. This sequence is quite different from the 10-bp sequence for Neisseria meningitidis uptake (nUS). During the sequence analysis of the superoxide dismutase gene (sodC) from N. meningitidis, Kroll et al.[ 1 Kroll J.S. et al. Natural genetic exchange between Haemophilus and Neisseria: intergeneric transfer of chromosomal genes between major human pathogens. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 12381-12385 Crossref PubMed Scopus (123) Google Scholar ]noted an inverted repeated sequence characteristic of a rho-independent terminator. Interestingly, this inverted repeat sequence included an HmUS. Protein sequence comparison of SodC from N. meningitidis and H. influenzae showed striking similarities, while the adjacent upstream N. meningitidis gene product showed a similarity to the homologous H. influenzae gene product more in keeping with its phylogenetic similarity. These findings suggest that the N. meningitidis sodC gene was obtained by horizontal gene transfer from Haemophilus. By scanning the partial meningococcal genome for homology to the HmUS, two additional loci with similarities to regions from H. influenzae were identified. One of these loci contained the HmUS within a protein coding region; this sequence is apparently not critical for this protein's activity, as it is not present in the corresponding Escherichia coli protein. By analysing the complete H. influenzae genome for potential nUS, DNA from a region near one of these sites was shown to have similarity to the DNA from the same region in N. meningitidis. These sequence comparisons suggest that transfer of DNA from H. influenzae to N. meningitidis has occurred. The potential for DNA transfer between these species during the colonization of the nasopharynx might lead to the acquisition of virulence or other factors that could result in a more pathogenic bacteria. An example of such a horizontal gene transfer may be found in H. influenzae biogroup aegyptius, the cause of septicaemia associated with the disease Brazilian purpuric fever, which acquired a highly virulent `meningococcal' phenotype.