A comparison of tyrosyl-tRNA synthetase of free-living and symbiotic Rhizobium species

Tyrosyl-tRNA synthetase was studied in partially purified extracts of free-living and symbiotic Rhizobium meliloti F-28 and Rhizobium japonicum 705. Certain differences in the properties of the enzymes were observed between the two rhizobial species and between free-living and symbiotic rhizobia. Differences were observed in optimal pH values and optimal magnesium and ATP concentrations. The optimum pH was 7.0 and 8.0 for the tyrosyl-tRNA synthetase of free-living and symbiotic R. meliloti, respectively. The optimum pH was 8.0 and 8.5 for the synthetase of free-living and symbiotic R. japonicum, respectively. The magnesium optimum concentration was 2.0 mM and 8.0 mM for the synthetase of free-living and symbiotic R. meliloti, respectively, and was 10.0 mM for both free-living and symbiotic R. japonicum systems. The optimum ATP concentration was 0.8 mM for the synthetases of both symbiotic rhizobia, but was 2.0 mM and 1.0 mM for free-living R. meliloti and R. "japonicum, respectively. Rhizobial sRNA was preferred for aminoacylation over yeast and calf liver sRNA. Enzyme extracts of the free-living and symbiotic rhizobia also were noted to differ in response to fractionation with ammonium sulfate. The highest tyrosyl-tRNA synthetase activity of free-living rhizobia was associated with the 55 to 75% ammonium sulfate fraction, whereas the highest synthetase activity of symbiotic rhizobia was associated with fractions of lower ammonium sulfate saturation. Tyrosyl-tRNA synthetase activity and protein and nucleic acid content of the symbiotic R. japonicum increased with the age of soybean nodules. These observations and possible implications of physiological importance were discussed.