Evidence for polynuclear iron (III) clusters in the root nodule bacterium, Rhizobium leguminosarum bv. viciae WSM710

Abstract

Cells of the root nodule bacterium Rhizobium leguminosarum bv. viciae WSM710 were cultured in a medium containing 20 μM 57Fe. Mossbauer spectra of the cells at 5.5 and 3.7 K indicated that the major form of iron present in the cells was in the form of polynuclear iron(III) clusters. At 5.5 K the spectral component associated with these clusters was in the form of a superposition of a broad feature (large magnetic hyperfine field distribution) and a doublet. On lowering the temperature of the cells to 3.7 K, the spectral component was transformed into resolved magnetic hyperfine field splitting which yielded a magnetic hyperfine field of 42.4 T when fitted with broad Lorentzian peaks. These spectral characteristics are typical of the hydrated iron(III) phosphate cores of several bacterioferritins. A small fraction (11%) of the Mossbauer spectral area of the cells was in the form of a doublet which yielded parameters (δ = 1.35 mm/s; ΔE(Q) = 3.15 mm/s) indicative of iron(II). The parameters are very similar to those of a spectral component previously observed in several other microbes and which has been associated with a 2.2 kDa oligomeric iron(II) carbohydrate phosphate