Ethylene and epoxyethane metabolism in methanotrophic bacteria: comparative genomics and physiological studies using Methylohalobius crimeensis

The genome of the methanotrophic bacterium <i>Methylohalobius crimeensis</i> strain 10Ki contains a gene cluster that encodes a putative coenzyme-M (CoM)-dependent pathway for oxidation of epoxyethane, based on homology to genes in bacteria that grow on ethylene and propylene as sole substrates. An alkene monooxygenase was not detected in the <i>M. crimeensis</i> genome, so epoxyethane is likely produced from co-oxidation of ethylene by the methane monooxygenase enzyme. Similar gene clusters were detected in about 10% of available genomes from aerobic methanotrophic bacteria, primarily strains grown from rice paddies and other wetlands. The sparse occurrence of the gene cluster across distant phylogenetic groups suggests that multiple lateral gene transfer events have occurred in methanotrophs. In support of this, the gene cluster in <i>M. crimeensis</i> was detected within a large genomic island predicted using multiple methods. Growth studies, reverse transcription-quantitative PCR (RT-qPCR) and proteomics were performed to examine the expression of these genes in <i>M. crimeensis</i>. Growth and methane oxidation activity were completely inhibited by the addition of >0.5% (v/v) ethylene to the headspace of cultures, but at 0.125% and below, the inhibition was only partial, and ethylene was gradually oxidized. The <i>etnE</i> gene encoding epoxyalkane:CoM transferase was strongly upregulated in ethylene-exposed cells based on RT-qPCR. Proteomics analysis confirmed that EtnE and nine other proteins encoded in the same gene cluster became much more predominant after cells were exposed to ethylene. The results suggest that ethylene is strongly inhibitory to <i>M. crimeensis</i>, but the bacterium responds to ethylene exposure by expressing an epoxide oxidation system similar to that used by bacteria that grow on alkenes. In the obligate methanotroph <i>M. crimeensis</i>, this system does not facilitate growth on ethylene but likely alleviates toxicity of epoxyethane formed through ethylene co-oxidation by particulate methane monooxygenase. The presence of predicted epoxide detoxification systems in several other wetland methanotrophs suggests that co-oxidation of ambient ethylene presents a stress for methanotrophic bacteria in these environments and that epoxyethane removal has adaptive value.