Bioconversion of natural gas to liquid fuel: opportunities and challenges

Genetic tools for the industrially promising methanotroph Methylomicrobium buryatense

Aerobic methanotrophs oxidize methane at ambient temperatures and pressures and are therefore attractive systems for methane-based bioconversions. In this work, we developed and validated genetic tools for Methylomicrobium buryatense, a haloalkaliphilic gammaproteobacterial (type I) methanotroph. M. buryatense was isolated directly on natural gas and grows robustly in pure culture with a 3-h doubling time, enabling rapid genetic manipulation compared to many other methanotrophic species. As a proof of concept, we used a sucrose counterselection system to eliminate glycogen production in M. buryatense by constructing unmarked deletions in two redundant glycogen synthase genes. We also selected for a more genetically tractable variant strain that can be conjugated with small incompatibility group P (IncP)-based broad-host-range vectors and determined that this capability is due to loss of the native plasmid. These tools make M. buryatense a promising model system for studying aerobic methanotroph physiology and enable metabolic engineering in this bacterium for industrial biocatalysis of methane.

Structure and protein-protein interactions of methanol dehydrogenase from Methylococcus capsulatus (Bath)

In the initial steps of their metabolic pathway, methanotrophic bacteria oxidize methane to methanol with methane monooxygenases (MMOs) and methanol to formaldehyde with methanol dehydrogenases (MDHs). Several lines of evidence suggest that the membrane-bound or particulate MMO (pMMO) and MDH interact to form a metabolic supercomplex. To further investigate the possible existence of such a supercomplex, native MDH from Methylococcus capsulatus (Bath) has been purified and characterized by size exclusion chromatography with multi-angle light scattering and X-ray crystallography. M. capsulatus (Bath) MDH is primarily a dimer in solution, although an oligomeric species with a molecular mass of ∼450–560 kDa forms at higher protein concentrations. The 2.57 Å resolution crystal structure reveals an overall fold and α₂β₂ dimeric architecture similar to those of other MDH structures. In addition, biolayer interferometry studies demonstrate specific protein–protein interactions between MDH and M. capsulatus (Bath) pMMO as well as between MDH and the truncated recombinant periplasmic domains of M. capsulatus (Bath) pMMO (spmoB). These interactions exhibit K_D values of 833 ± 409 nM and 9.0 ± 7.7 μM, respectively. The biochemical data combined with analysis of the crystal lattice interactions observed in the MDH structure suggest a model in which MDH and pMMO associate not as a discrete, stoichiometric complex but as a larger assembly scaffolded by the intracytoplasmic membranes.