Comparison of DNA Polymerase of Rhizobium meliloti and Alfalfa Bacteroids

DNA dependent-DNA polymerase activity was established and partially purified from extracts of cultured Rhizobium meliloti, F-28, and nodule bacteroids (R. meliloti, F-28) of alfalfa plants (Medicago sativa). Polymerase activity in the partially purified fractions showed characteristic dependence on Mg(2+), DNA, and a full complement of deoxyribonucleoside triphosphates. DNase activity, preference of "activated" double strand DNA, and inhibition by p-chloromercuribenzoate and MnCl(2) were responses common to both systems. The two systems however did exhibit some differences in pH, Mg(2+), and primer optima. Polymerase activity in crude extracts of the cultured bacteria was more stable and had 10- to 18-fold greater specific activity than the bacteroid extracts. Preliminary measurements of specific DNA polymerase activity in crude extracts of cultured Rhizobium japonicum were not significantly higher than that in the crude extracts of soybean nodule bacteroids. A possible correlation between DNA synthesis and the successful establishment of rhizobia-legume symbiosis is discussed.