Stimulation of transposition of the Mycobacterium tuberculosis insertion sequence IS6110 by exposure to a microaerobic environment

The Mycobacterium tuberculosis-specific insertion sequence IS6110/986 has b een widely used as a probe because of the multiple polymorphism observed am ong different strains. To investigate transposition of IS6110 a series of a rtificially constructed composite transposons containing IS6110 and a kanam ycin resistance marker were constructed. The composite transposons were ins erted into a conditionally replicating, thermosensitive, Escherichia coli-m ycobacterial shuttle vector and introduced into M. smegmatis mc(2)155, Lawn s of transformants were grown at the permissive temperature on kanamycin-su pplemented agar and subsequently prevented from further growth by shifting to the nonpermissive temperature. Under normal atmospheric conditions, kana mycin-resistant papillae appeared after only about 5-6 weeks of incubation. However, these events were not associated with transposon mobilization. In contrast, lawns that were exposed to a 48 h microaerobic shock generated k anamycin-resistant papillae after only 6-14 days. These events were generat ed by conservative transposition of the IS6110 composite transposon into th e M. smegmatis chromosome, with loss of the shuttle vector. In common with other IS3 family elements, transposition of IS6110 is thought to be control led by translational frameshifting. However, we were unable to detect any s ignificant frameshifting within the putative frameshifting site of IS6110 a nd the level of frameshifting was not affected by microaerobic incubation. The finding that transposition of IS6110 is stimulated by incubation at red uced oxygen tensions may be relevant to transposition of IS6110 in M. tuber culosis harboured within TB lesions.