IS6110 TRANSPOSITION AND EVOLUTIONARY SCENARIO OF THE DIRECT REPEAT LOCUS IN A GROUP OF CLOSELY-RELATED MYCOBACTERIUM-TUBERCULOSIS STRAINS

In recent years, various polymorphic loci and multicopy insertion elem ents have been discovered in the Mycobacterium tuberculosis genome, su ch as the direct repent (DR) locus, the major polymorphic tandem repea ts, the polymorphic GC-rich repetitive sequence, IS6110, and IS1081, T hese, especially IS6110 and the DR locus, have been widely used as gen etic markers to differentiate;ll, tuberculosis isolates and will conti nue to be so used, due to the conserved nature of the genome of M. tub erculosis. However, little is known about the processes involved in ge nerating these or of their relative rates of change. Without an unders tanding of the biological characteristics of these genetic markers, it is difficult to use them to their full extent for understanding the p opulation genetics and epidemiology of A. tuberculosis, To address the se points, we identified a cluster of 7 isolates in a collection of 10 1 clinical isolates and in investigated them with various polymorphic genetic markers, which indicated that they were highly related to each other. This cluster provided a model system for the study of IS6110 t ransposition, evolution at the DR locus, and the effects of these on t he determination of evolutionary relationships among M. tuberculosis s trains. Our results suggest that IS6110 restriction fragment length po lymorphism patterns are useful in grouping closely related isolates to gether; however, they can be misleading if used for making inferences about the evolutionary relationships between closely related isolates. DNA sequence analysis of the DR loci of these isolates revealed an ev olutionary scenario, which, complemented with the information from IS6 110, allowed a reconstruction of the evolutionary steps and relationsh ips among these closely related isolates, Loss of the IS6110 copy in t he DR locus was noted, and the mechanisms of this loss are discussed.