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

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). BL2 and AR4 are closely related methanotroph species that oxidize methane via sMMO only. However, is an obligate methanotroph, while is a facultative methanotroph able to grow on several multicarbon substrates in addition to methane. We constructed transcriptional fusions of the promoters of and to a promoterless 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 promoter of the facultative methanotroph was either transcriptionally downregulated or repressed by any growth substrate other than methane in the genetic background of ; (2) Growth on methane alone upregulated the promoter of in its native background but not in the obligate methanotroph ; (3) The promoter of was constitutive in both organisms regardless of the growth substrate, but with much lower promoter activity than the promoter of . These results support a conclusion that a different mode of transcriptional regulation of sMMO contributes to the facultative lifestyle of compared to the obligate methanotroph .