The ICEXTD of Azoarcus sp. CIB, an integrative and conjugative element with aerobic and anaerobic catabolic properties.

Integrative and conjugative elements (ICE) play a major role in aerobic degradation of aromatic compounds, but they have not yet been shown to be involved in anaerobic degradation. We have characterized here the ICEXTD element which endows to the beta-proteobacterium Azoarcus sp. CIB with the ability to utilize aromatic hydrocarbons. The core region of ICEXTD , which shows a remarkable synteny with that of ICEclc-like elements, allows its own intracellular and intercellular mobility. ICEXTD integrates at the tRNAGly of the host chromosome, but it can also excise to produce a ready to transfer circular form. The adaptation modules of ICEXTD represent a unique combination of gene clusters for aerobic (tod genes) and anaerobic (bss-bbs and mbd genes) degradation of certain aromatic hydrocarbons, e.g., toluene, m-xylene and cumene. Transfer of ICEXTD to other Azoarcus strains, e.g., A. evansii, confers them the ability to degrade aromatic hydrocarbons both aerobically and anaerobically. Interestingly, ICEXTD allows Cupriavidus pinatubonensis, a bacterium unable to degrade anaerobically aromatic compounds, to grow with m-xylene under anoxic conditions. Thus, ICEXTD constitutes the first mobile genetic element able to expand the catabolic abilities of certain bacteria for the removal of aromatic hydrocarbons either in the presence or absence of oxygen.