, 2005; Erb et al., 2007, 2009; Berg & Ivanovsky, 2009; Peyraud et al., 2009; Alber, 2011; Khomyakova et al. 2011). It is interesting that some intermediates of these assimilatory pathways, for example malate and glyoxylate, are also intermediates in the serine cycle and as such may Selleck PD0325901 afford easy coupling with utilization of the serine cycle. Identification of

acetate utilization pathways in methanotrophs, however, has been challenging. For example, early enzymatic work on M. silvestris found no evidence for the key enzymatic activities in the glyoxylate cycle, i.e., isocitrate lyase and malate synthase (Dunfield et al., 2003; Theisen et al., 2005). Genomic analyses, however, show that genes encoding for these enzymes are present (Chen et al., 2010a). Subsequent deletion of the gene encoding for isocitrate lyase severely limited growth of M. silvestris Venetoclax mouse on acetate, and abolished it on methane (Crombie & Murrell, 2011). As discussed by the authors, such data suggest that the glyoxylate shunt may be vital to M. silvestris for regeneration of glyoxylate in the serine cycle used for carbon assimilation from C1 compounds as well as from C2 compounds. These findings also suggest that this microorganism may have multiple mechanisms to utilize multicarbon

compounds, as growth still occurred on acetate when the gene encoding for isocitrate lyase was deleted. However, homologs of known key genes of ethylmalonyl-CoA, citramalate, and methylaspartate pathways for carbon assimilation from acetate are not readily apparent in the genome sequence of M. silvestris. In contrast,

phylogenetically closely related methylotrophs such as the alphaproteobacterium M. extorquens AM1 were often shown to utilize the coupled serine and ethylmalonyl-CoA pathways for growth (Peyraud et al., 2009; Ŝmejkalová et al., 2010). Preliminary analysis of publicly available genome sequences Cyclic nucleotide phosphodiesterase of obligate methanotrophs [i.e. Alphaproteobacteria Methylosinus trichosporium OB3b (Stein et al., 2010), Methylocystis sp. strain ATCC 49242 (Stein et al., 2011), Gammaproteobacteria M. capsulatus Bath (Ward et al., 2004), Methylobacillus flagellatus KT (Chistoserdova et al., 2007), Methylobacter tundripaludum SV96, Methylomicrobium album BG8, Methylomonas methanica MC09, as well as Candidatus Methylomirabilis oxyfera (Ettwig et al., 2010) and Methylacidiphilum infernorum V4 (Hou et al., 2008)], indicates that the key genes of the ethylmalonyl-CoA pathway (Fig. 3) are only present in the two alphaproteobacterial methanotrophs that were sequenced so far, and are found in synteny in the Methylocystis strain. Further, no evidence was observed for the presence of the set of key genes defining citramalate (Fig. 4) or methylaspartate pathways (Fig. 5) for multicarbon assimilation in any methanotroph for which a genome sequence is available. At present, however, such observations should be treated with caution. First, sequence information is still lacking for some reactions (e.g.