Fuel oxygenates such as Me and Et tert-Bu ether (MTBE and ETBE, resp.) are degraded only by a limited no. of bacterial strains. The aerobic pathway is generally thought to run via tert-Bu alc. (TBA) and 2-hydroxyisobutyrate (2-HIBA), whereas further steps are unclear. We have now demonstrated for the newly isolated b-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 enzymic step. In these strains, growth on substrates contg. the tert-Bu 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 anal. identified it as the small subunit of isobutyryl-CoA (CoA) mutase (ICM; EC 22.214.171.124), which is a cobalamin-contg. 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 exts. of induced cells of strains L108 and L10, transforming 2-HIBA into 3-hydroxybutyrate in the presence of CoA and ATP. The physiol. and evolutionary aspects of this novel pathway involved in MTBE and ETBE metab. are discussed.