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

Cells of the root nodule bacterium Rhizobium leguminosarum bv. viciae WSM71 0 were cultured in a medium containing 20 mu M Fe-57. 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 supe rposition of a broad feature (large magnetic hyperfine field distribution) and a doublet. On lowering the temperature of the cells to 3.7 K, the spect ral component was transformed into resolved magnetic hyperfine field splitt ing which yielded a magnetic hyperfine field of 42.4 T when fitted with bro ad Lorentzian peaks. These spectral characteristics are typical of the hydr ated iron(III) phosphate cores of several bacterioferritins. A small fracti on (11%) of the Mossbauer spectral area of the cells was in the form of a d oublet which yielded parameters (delta = 1.35 mm/s; Delta E-Q = 3.15 mm/s) indicative of iron(II). The parameters are very similar to those of a spect ral component previously observed in several other microbes (R. Bohnke and B.F. Matzanke (1995) BioMetals 8, 223-230) and which has been associated wi th a 2.2 kDa oligomeric iron(II) carbohydrate phosphate.