Plasmids, Phages, and Gene Transfer in Methanogenic Bacteria. In: Sebald M, editor. Genetics and Molecular Biology of Anaerobic Bacteria. New York: Springer; 1993. pp. 1-12. [DOI: 10.1007/978-1-4615-7087-5_1]

A Shuttle-Vector System Allows Heterologous Gene Expression in the Thermophilic Methanogen Methanothermobacter thermautotrophicus ΔH

Thermophilic Methanothermobacter spp. are used as model microbes to study the physiology and biochemistry of the conversion of molecular hydrogen and carbon dioxide into methane (i.e., hydrogenotrophic methanogenesis). Yet, a genetic system for these model microbes was missing despite intensive work for four decades. Here, we report the successful implementation of genetic tools for Methanothermobacter thermautotrophicus ΔH. We developed shuttle vectors that replicated in Escherichia coli and M. thermautotrophicus ΔH. For M. thermautotrophicus ΔH, a thermostable neomycin resistance cassette served as the selectable marker for positive selection with neomycin, and the cryptic plasmid pME2001 from Methanothermobacter marburgensis served as the replicon. The shuttle-vector DNA was transferred from E. coli into M. thermautotrophicus ΔH via interdomain conjugation. After the successful validation of DNA transfer and positive selection in M. thermautotrophicus ΔH, we demonstrated heterologous gene expression of a thermostable β-galactosidase-encoding gene (bgaB) from Geobacillus stearothermophilus under the expression control of four distinct synthetic and native promoters. In quantitative in-vitro enzyme activity assay, we found significantly different β-galactosidase activity with these distinct promoters. With a formate dehydrogenase operon-encoding shuttle vector, we allowed growth of M. thermautotrophicus ΔH on formate as the sole growth substrate, while this was not possible for the empty-vector control.

Regulated gene expression in methanogens

Methanogens form a very large and diverse group of microorganisms within the domain of the Archaea. Energy for their growth is obtained by the reduction of a variety of substrates into methane. Several genes, coding for enzymes involved in methanogenesis or in the central metabolism have been cloned and studied. The molecular features of their expression signals have been compared with bacterial and eukaryal expression signals. This indicated that the transcription process is an intermediate form between the specific processes known in Bacteria and Eukarya. The translation system that is used in Archaea is very similar with the process in Bacteria. Although the molecular features of genes and expression signals in Archaea are well-studied, investigations on the regulation of gene expression in these organisms are very scarce. In order to give insight into molecular regulatory mechanisms in methanogens, the current knowledge of regulated systems in methanogens will be reviewed in this manuscript.