No abstract

An aerobic bacterium (Mycobacterium sp. strain ELW1) that utilizes 2-methylpropene (isobutylene) as a sole source of carbon and energy was isolated and characterized. Strain ELW1 grew on 2-methylpropene (growth rate ‫؍‬ 0.05 h ؊1 ) with a yield of 0.38 mg (dry weight) mg 2-methylpropene ؊1 . Strain ELW1 also grew more slowly on both cis-and trans-2-butene but did not grow on any other C 2 to C 5 straight-chain, branched, or chlorinated alkenes tested. Resting 2-methylpropene-grown cells consumed ethene, propene, and 1-butene without a lag phase. Epoxyethane accumulated as the only detected product of ethene oxidation. Both alkene consumption and epoxyethane production were fully inhibited in cells exposed to 1-octyne, suggesting that alkene oxidation is initiated by an alkyne-sensitive, epoxide-generating monooxygenase. Kinetic analyses indicated that 1,2-epoxy-2-methylpropane is rapidly consumed during 2-methylpropene degradation, while 2-methyl-2-propen-1-ol is not a significant metabolite of 2-methylpropene catabolism. Degradation of 1,2-epoxy-2-methylpropane by 2-methylpropene-grown cells led to the accumulation and further degradation of 2-methyl-1,2-propanediol and 2-hydroxyisobutyrate, two sequential metabolites previously identified in the aerobic microbial metabolism of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA). Growth of strain ELW1 on 2-methylpropene, 1,2-epoxy-2-methylpropane, 2-methyl-1,2-propanediol, and 2-hydroxyisobutyrate was fully inhibited when cobalt ions were omitted from the growth medium, while growth on 3-hydroxybutyrate and other substrates was unaffected by the absence of added cobalt ions. Our results suggest that, like aerobic MTBE-and TBA-metabolizing bacteria, strain ELW1 utilizes a cobalt/cobalamin-dependent mutase to transform 2-hydroxyisobutyrate. Our results have been interpreted in terms of their impact on our understanding of the microbial metabolism of alkenes and ether oxygenates.T he compound 2-methylpropene (isobutylene) is one of four butene (C 4 H 8 ) isomers and is the simplest branched alkene. Annually, in excess of 10 million tons of this gas are produced worldwide, mainly from cracking of petroleum. 2-Methylpropene is used as feedstock for high-volume petrochemicals, including ether fuel oxygenates, such as methyl tert-butyl ether (MTBE); high-octane gasoline blending components, such as 2,2,4-trimethylpentane (isooctane); and butyl rubber (1). The industrial importance of 2-methylpropene has stimulated interest in its biological production as an alternative to nonrenewable petroleum sources. Many microorganisms that can generate this gas have been identified (2, 3), including MTBE-metabolizing bacteria (4). It can also be produced directly by several enzymes (5-8). In contrast to these microbial and enzymatic production routes, little is known about the microbial degradation of 2-methylpropene, and no microorganisms have previously been reported to grow on this gas.From their outset, studies of aerobic microbial metabolism of gaseous alkenes have f...