Author(s): Rohwerder T, Breuer U, Benndorf D, Lechner U, Mller RH
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Abstract Fuel oxygenates such as methyl and ethyl tert-butyl ether (MTBE and ETBE, respectively) are degraded only by a limited number of bacterial strains. The aerobic pathway is generally thought to run via tert-butyl alcohol (TBA) and 2-hydroxyisobutyrate (2-HIBA), whereas further steps are unclear. We have now demonstrated for the newly isolated beta-proteobacterial strains L108 and L10, as well as for the closely related strain CIP I-2052, that 2-HIBA was degraded by a cobalamin-dependent enzymatic step. In these strains, growth on substrates containing the tert-butyl moiety, such as MTBE, TBA, and 2-HIBA, was strictly dependent on cobalt, which could be replaced by cobalamin. Tandem mass spectrometry identified a 2-HIBA-induced protein with high similarity to a peptide whose gene sequence was found in the finished genome of the MTBE-degrading strain Methylibium petroleiphilum PM1. Alignment analysis identified it as the small subunit of isobutyryl-coenzyme A (CoA) mutase (ICM; EC 18.104.22.168), which is a cobalamin-containing carbon skeleton-rearranging enzyme, originally described only in Streptomyces spp. Sequencing of the genes of both ICM subunits from strain L108 revealed nearly 100\% identity with the corresponding peptide sequences from M. petroleiphilum PM1, suggesting a horizontal gene transfer event to have occurred between these strains. Enzyme activity was demonstrated in crude extracts of induced cells of strains L108 and L10, transforming 2-HIBA into 3-hydroxybutyrate in the presence of CoA and ATP. The physiological and evolutionary aspects of this novel pathway involved in MTBE and ETBE metabolism are discussed.
This article was published in Appl Environ Microbiol
and referenced in Journal of Bioremediation & Biodegradation