Antigen diversity in the bacterium B. hermsii through "somatic" mutations in rearranged vmp genes

B. hermsii counters host immunity with multiphasic antigenic variation. This is conferred by interplasmidic and intraplasmidic rearrangements of vmp genes. In several independent events, activation of a silent vmp gene through intraplasmidic deletions but not interplasmidic recombinations was followed by the appearance at its 5' end of multiple mutations that were not present in the silent gene. The prevalence of mutant alleles in postwitch populations increased during infections. Differences between the silent and expressed genes were at the same nucleotides at which vmp pseudogenes differed, suggesting these were templates for postswitch gene conversions. The mechanism of this bacterium to generate diversity, namely, intramolecular deletions followed by mutations in the rearranged gene, mirrors the strategy used by vertebrate hosts to eliminate it.