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Eikenella corrodens belongs to a group of periodontopathogenic bacteria and forms unique corroding colonies on solid medium due to twitching motility. It is believed that an N-acetyl-d-galactosamine (GalNAc)-specific lectin on the cell surface contributes significantly to its pathogenicity and can be estimated by its haemagglutination (HA) activity. Recently, a plasmid, pMU1, from strain 1073 has been found; this plasmid affects pilus formation and colony morphology. To identify the gene involved in these phenomena, ORF 4 and ORFs 5–6 on pMU1 were separately subcloned into a shuttle vector, and the resultant plasmids were introduced into E. corrodens 23834. Transformants with the ORF 4 gene, which is identified to be a homologous gene of the type IV pilin gene-specific recombinase, lost their pilus structure and formed non-corroding colonies on a solid medium, whereas transformants with ORFs 5–6 exhibited the same phenotype as the host strain 23834. Southern analysis showed that the introduction of the ORF 4 gene into strain 23834 resulted in genomic recombination at the type IV pilin gene locus. The hybridization pattern of these transformants was similar to that of strain 1073. These results suggest that ORF 4 on pMU1 encodes a site-specific recombinase and causes genomic recombination of the type IV pilin gene locus. Furthermore, the introduction of ORF 4 into strain 23834 increased GalNAc-specific HA activity to a level equivalent to that of strain 1073. Although the morphological colony changes and loss of pilus structure are also observed in phase variation, genomic recombination of the type IV pilin gene locus did not occur in these variants. Moreover, an increase was not observed in the GalNAc-specific HA activity of these variants. These results suggested that the loss of pilus structure, the morphological change in colonies and the increase in HA activity due to plasmid pMU1 might be caused by a mechanism that differs from phase variation, such as a genomic recombination of the type IV pilin gene locus.

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