Oxidation pathways in methylotrophs

An extremely oligotrophic bacterium, Rhodococcus erythropolis N9T-4, isolated from crude oil

Rhodococcus erythropolis N9T-4, which was isolated from crude oil, showed extremely oligotrophic growth and formed its colonies on a minimal salt medium solidified using agar or silica gel without any additional carbon source. N9T-4 did not grow under CO(2)-limiting conditions but could grow on a medium containing NaHCO(3) under the same conditions, suggesting that the oligotrophic growth of N9T-4 depends on CO(2). Proteomic analysis of N9T-4 revealed that two proteins, with molecular masses of 45 and 55 kDa, were highly induced under the oligotrophic conditions. The primary structures of these proteins exhibited striking similarities to those of methanol: N,N'-dimethyl-4-nitrosoaniline oxidoreductase and an aldehyde dehydrogenase from Rhodococcus sp. These enzyme activities were three times higher under oligotrophic conditions than under n-tetradecane-containing heterotrophic conditions, and gene disruption for the aldehyde dehydrogenase caused a lack of growth on the minimal salt medium. Furthermore, 3-hexulose 6-phosphate synthase and phospho-3-hexuloisomerase activities, which are key enzymes in the ribulose monophosphate pathway in methylotrophic bacteria, were detected specifically in the cell extract of oligotrophically grown N9T-4. These results suggest that CO(2) fixation involves methanol (formaldehyde) metabolism in the oligotrophic growth of R. erythropolis N9T-4.

Optimization and maintenance of soluble methane monooxygenase activity in Methylosinus trichosporium OB3b

Soluble methane monooxygenase (sMMO) maximization studies were carried out as part of a larger effort directed towards the development and optimization of an aqueous phase, multistage, membrane bioreactor system for treatment of polluted groundwater. A modified version of the naphthalene oxidation assay was utilized to determine the effects of methane:oxygen ratio, nutrient supply, and supplementary carbon sources on maximizing and maintaining sMMO activity in Methylosinus trichosporium OB3b. Methylosinus trichosporium OB3b attained peak sMMO activity (275-300 nmol of naphthol formed h-1 mg of protein-1 at 25 degrees C) in early stationary growth phase when grown in nitrate mineral salts (NMS) medium. With the onset of methane limitation however, sMMO activity rapidly declined. It was possible to define a simplified nitrate mineral salts (NMS) medium, containing nitrate, phosphate and a source of iron and magnesium, which allowed reasonably high growth rates (mu max 0.08 h-1) and growth yields (0.4-0.5 g cells/g CH4) and near maximal activities of sMMO. In long term batch culture incubations sMMO activity reached a stable plateau at approximately 45-50% of the initial peak level and this was maintained over several weeks. The addition of d-biotin, pyridoxine, and vitamin B12 (cyanocobalamin) increased the activity level of sMMO in actively growing methanotrophs by 25-75%. The addition of these growth factors to the simplified NMS medium was found to increase the plateau sMMO level in long term batch cultures up to 70% of the original peak activity.