Enzymatic synthesis of deoxyribonucleic acid. XXVI. Physical and chemical studies of a homogeneous deoxyribonucleic acid polymerase

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The pyrophosphate (PP i ) exchange reaction catalyzed by Escherichia coli DNA polymerase is identical with the polymerization reaction in its requirements for a template, strict specificity in base pairing, and a 3'-hydroxyl-terminated primer strand. However, in contrast to polymerization, appreciable exchange is obtained in the absence of a full complement of deoxyribonucleoside triphosphates. Inhibition of synthesis by PP i as measured by the difference between PP i release and nucleotide incorporation from deoxyribonucleoside triphosphates, can be accounted for by PP i exchange. The PP i exchange reaction appears to represent the removal by PP i of the newly incorporated nucleotide prior to its complete stabilization by the entry of the next triphosphate.The degradation of DNA by PP i (pyrophosphorolysis) appears to differ from the PP i exchange reaction in that it is inhibited by deoxyribonucleoside triphosphates, attains a steady state plateau, and has a lower pH optimum and a slower rate. Some of these differences may depend on the requirement that progressive pyrophosphorolysis places on progressive movement of the DNA chain relative to the enzyme.The results are discussed in relation to the mechanism of the PP i exchange reaction and to all polymerase functions in a model with a single active center for the enzyme.

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When tomatoes were planted in steamed soil inoculated with spores of Didymella lycopersici Kleb., a large proportion became infected but very few became infected in unsteamed soil similarly inoculated. In soil treated with formalin and subsequently inoculated the number of diseased plants was about half that in steamed soil. Reinfecting steamed soil with soil micro-organisms before inoculation with D. lycopersici reduced the amount of disease. Covering steamed soil with a layer of maiden loam filtered out a large proportion of D. lycopersici spores applied in water suspension and reduced the amount of disease. Addition of stable manure either before or after steaming did not affect the number of diseased plants Steamed soil remained susceptible to infection by D. lycopersici for 4 months