Hemoglobin in Frankia, a Nitrogen-Fixing Actinomycete

Frankia strain CcI3 grown in culture produced a hemoglobin which had optical absorption bands typical of a hemoglobin and a molecular mass of 14.1 kDa. Its equilibrium oxygen binding constant was 274 nM, the oxygen dissociation rate constant was 56 s(−1), and the oxygen association rate constant was 206 μM(−1) s(−1)

Hemoglobin in five genetically diverse Frankia strains

Five strains of Frankia were selected to represent a wide range of genetic diversity and examined for presence of hemoglobin. All five strains produced hemoglobin when grown on media without (N) or with (+N) combined nitrogen. This indicates that hemoglobin is common in Frankia and is not directly associated with nitrogen fixation. Frankia strain EAN1pec was examined in more detail. It showed greater hemoglobin concentration when grown at 2% O2 than at 20% O2 in the N treatment but no effect of oxygen on hemoglobin concentration in the +N treatment. At both oxygen levels, it produced substantially more biomass in +N than in N culture. It also produced significantly more biomass when the medium contained 0.2% CO2 than in the absence of CO2. The molecular mass of the hemo- globin as determined by size exclusion chromatography was 13.4 ± 0.2 kDa (mean ± SE, n = 3) and is consistent with that of a truncated hemoglobin. The hemoglobin had absorption spectra that were typical of a hemoglobin. The oxygen dissociation rate constants for the hemoglobin were 131.2 ± 5.8 s1 for N culture and 166 ± 8.2 s1 for +N culture. These rapid rates are consistent with a function in facilitated diffusion of oxygen.Key words: Frankia, hemoglobin, truncated hemoglobin