Cultures of M. capsulatus Bath were grown in NMS (Poret-Peterson et al., 2008), harvested at mid-exponential phase, washed and resuspended to c. 108 cells mL−1 in NMS medium (CH4-only treatment) or NMS medium amended with 0.5 mM sodium nitroprusside (i.e. CH4+SNP treatment)

or 0.25 mM NaNO2 (i.e. CH4+NO2− treatment). Resuspended M. capsulatus Bath cells were exposed to 0.25 mM NaNO2 or 0.5 mM SNP Trametinib supplier for 4 h after which steady-state mRNA levels of norC, norB, cytS, cytL, haoA, rpoB, and nirB were measured by fluorescent real-time PCR (qPCR; iCycler, BioRad). Cells retained activity in the presence of NaNO2 and SNP as measured by rates of CH4 consumption and CO2 production. RNA was extracted using the FastRNA Pro Blue

kit (Qbiogene, Irvine, CA) and converted to cDNA using Superscript III (Invitrogen). Primer sets, qPCR conditions, and product quality assessment were the same as reported previously (Poret-Peterson et al., 2008). N2O production was measured by GC (Shimadzu GC-8A; Hayesep Q column, Alltech) after 24 and 48 h in incubations of harvested and resuspended M. capsulatus Bath cells (at c. 108 cells mL−1) in NMS amended with 0.25 mM NaNO2, 0.5 mM SNP, or 5 mM (NH4)2SO4 in the presence of CH4. To test whether both ammonium and NOx were required to induce N2O production, M. capsulatus Bath was incubated with NaNO2 or SNP plus 5 mM (NH4)2SO4, which induces expression of haoAB and Lumacaftor in vitro cytS genes (Poret-Peterson et al., 2008). In these assays, cells retained >90% viability following centrifugation and resuspension as measured by comparing rates of formate oxidation to CO2 between cells from fresh culture and those resuspended into fresh NMS medium (data not shown). The methanotrophs M. album ATCC 33003, M. sporium ATCC 35069, and Methylocystis sp. Rockwell were previously shown to oxidize NH3 and NH2OH to NO2−, whereas this

activity was not detected in the soil strain, M. methanica Rubra (Nyerges & Stein, 2009). None of these methanotrophs were previously known to harbor genes encoding putative NH2OH oxidoreductases. Analysis by Southern blot revealed that the genomic DNAs from M. album ATCC 33003, M. album BG8, and M. capsulatus Bath produced positive PD184352 (CI-1040) hybridization signals to 32P-labeled fragments of haoA genes, whereas DNAs from the other strains did not (verified by genome sequences; see Table 2). Although Methylocystis sp. Rockwell did not show positive hybridization to haoA gene probes from M. capsulatus Bath or M. album ATCC 33003, examination of its genome sequence revealed haoAB homologues (Tables 1 and 2). No haoAB homologues were found in the genome of M. trichosporium OB3b, suggesting that this bacterium, and likely M. sporium ATCC 35069, converts NH2OH to nitrite through a different pathway (Stein et al., 2010). Analysis of the haoA genes and flanking sequence obtained from M.