Mapping of IS6110 flanking regions in clinical isolates of Mycobacterium tuberculosis demonstrates genome plasticity

Southern hybridization was used in combination with IS6110 insertion-locus- specific probes in a comparative study to determine the structure of chromo somal domains flanking IS6110 elements in clinical isolates of Mycobacteriu m tuberculosis. The resulting restriction fragment length polymorphism (RFL P) data demonstrated three mutational mechanisms responsible for the polymo rphisms observed: IS6110 insertion, chromosomal mutation and deletion. The frequency of IS6110 insertion within many of the chromosomal regions demons trates that preferential integration regions are common in M. tuberculosis. Mapping the IS6110 insertion positions and chromosomal deletions in relati on to the M. tuberculosis H37Rv and M. bovis BCG genome sequences reveals n umerous disruptions of predicted open reading frames (ORFs). A phylogenetic tree, based on the mutational data, showed a number of independently evolv ing lineages of M. tuberculosis, while analysis of the mutational events oc curring at each branch point suggests both divergent and convergent evoluti on. A significant positive correlation was demonstrated between the mutatio n rate and the frequency of occurrence of different isolates in families of strains, suggesting that evolution may impact on strain 'fitness' or that strain proliferation may increase the chance of mutation. We conclude that the genome of clinical isolates of M. tuberculosis continues to evolve.