Unlike M1, AbM4 does not require exogenous coenzyme M for growth as it contains the full complement of CoM biosynthesis genes. A complete set of cobalamin biosynthesis genes are also present, although they are scattered throughout the genome, rather than being clustered together as in M1. Unusually, the DNA-directed RNA polymerase technical support B�� and B��� subunits of AbM4 are joined, a feature only previously observed in some thermophilic Archaea [54]. Table 6 Restriction-modification system genes unique to AbM4 Insights from the Genome Sequence Overall, the genome of Methanobrevibacter sp. AbM4 is comparable to that of M. ruminantium M1 suggesting that the hydrogenotrophic, methane-forming metabolism of these rumen methanogens is highly analogous.

The differences observed between AbM4 and M1 in the abundance of adhesin-like proteins indicates AbM4 invests less of its genetic resources on external interactions with its environment. The broader repertoire of cofactor and coenzyme biosynthetic genes of AbM4 also indicates that it is likely to be less dependent on other rumen microbes for the supply of cofactors for growth and survival in the rumen. These features suggest that AbM4 occupies a ruminal niche slightly different from that of M1. Although AbM4 does not constitute large methanogen populations, it is widely distributed in ruminant species under different rumen and gut conditions. The conserved nature of the AbM4 genes encoding these methanogenesis functions, as well as those encoding other potential targets for methane mitigation, indicates that AbM4 will be amenable to inhibition by small molecule inhibitors and vaccine-based methane mitigation technologies targeting these genes.

Acknowledgements The AbM4 genome sequencing project was funded by the New Zealand Agricultural Greenhouse Gas Research Centre as part of the Methane Objective 1.3: Genomic identification of universal targets for methanogen inhibition. Methanobrevibacter sp. AbM4 cultures were made available for genome sequencing by the New Zealand Pastoral Greenhouse Gas Research Consortium. Electron microscopy was conducted with the assistance of the Manawatu Microscopy and Imaging Centre at Massey University, Palmerston North, New Zealand.
The Genomic Encyclopedia of Bacteria and Archaea (GEBA) project was established as a collaboration between the DOE Joint Genome Institute (JGI, Walnut Creek, CA) and a Biological Resource Center (BRC), the German Collection of Microorganisms and Cell Cultures (DSMZ).

The goal of GEBA is to obtain reference genomes GSK-3 that more broadly cover the evolutionary diversity of prokaryotes. Once sequencing and annotation are completed, GEBA genomes are submitted to the INSDC databases and made available to the public in the Integrated Microbial Genomes system [1].