Differential Transcriptional Activation of Genes Encoding Soluble Methane Monooxygenase in a Facultative Versus an Obligate Methanotroph

Abstract

Methanotrophs are a specialized group of bacteria that can utilize methane (CH4) as a sole energy source. A key enzyme responsible for methane oxidation is methane monooxygenase (MMO), of either a soluble, cytoplasmic type (sMMO), or a particulate, membrane-bound type (pMMO). Methylocella silvestris BL2 and Methyloferula stellata AR4 are closely related methanotroph species that oxidize methane via sMMO only. However, Methyloferula stellata is an obligate methanotroph, while Methylocella silvestris is a facultative methanotroph able to grow on several multicarbon substrates in addition to methane. We constructed transcriptional fusions of the mmo promoters of Methyloferula stellata and Methylocella silvestris to a promoterless gfp in order to compare their transcriptional regulation in response to different growth substrates, in the genetic background of both organisms. The following patterns were observed: (1) The mmo promoter of the facultative methanotroph Methylocella silvestris was either transcriptionally downregulated or repressed by any growth substrate other than methane in the genetic background of Methylocella silvetris; (2) Growth on methane alone upregulated the mmo promoter of Methylocella silvetris in its native background but not in the obligate methanotroph Methyloferula stellata; (3) The mmo promoter of Methyloferula stellata was constitutive in both organisms regardless of the growth substrate, but with much lower promoter activity than the mmo promoter of Methylocella silvetris. These results support a conclusion that a different mode of transcriptional regulation of sMMO contributes to the facultative lifestyle of Methylocella silvetris compared to the obligate methanotroph Methyloferula stellata.

Sulfur and methane oxidation by a single microorganism

“…The high abundance of Mmo proteins in thiosulfate-grown cells was unexpected since these cells did not consume oxygen in the microrespirometry experiments when provided with methane as the sole substrate ( Table 2 ). Expression of some genes encoding MMO in the absence of methane was observed previously ( 19 , 98 ). The presence of these proteins in all of the conditions tested might be an adaptive regulatory trait for rapid induction of methane oxidation in response to dynamic fluxes of methane in wetland environments.…”

Novel facultative Methylocella strains are active methane consumers at terrestrial natural gas seeps

“…Methylocella sp. PC1 and PC4 from Pipe Creek exhibited faster growth rates on methane and propane (up to 0.04 h −1 , Table 2) compared to those previously reported for Methylocella (0.01–0.02 h −1 on methane and 0.005–0.015 h −1 on propane) [49, 102, 103]. As with Methylocella silvestris BL2 [49], they grew under methane and propane simultaneously and could also utilise non-gaseous multicarbon compounds, including acetate, pyruvate and succinate (Additional file 1: Table S1).…”

Facultative methanotrophs – diversity, genetics, molecular ecology and biotechnological potential: a mini-review

“…It was suggested that use of acetate by facultative methanotrophs might be a survival strategy when the supply of methane is intermittent, and reports have shown that transcription of the pMMO genes continues under these conditions [39, 153–155]. In Methylocella silvestris , acetate repressed sMMO gene transcription, at least at the concentration tested (5 mM) [159, 160], although this was not the case for propane [44]. Recently, with the availability of additional genome sequences, Farhan Ul Haque et al [161] designed improved Methylocella -specific mmoX primers, which could be used to quantify Methylocella -like sMMO gene sequences in environmental samples.…”