Deoxyribonucleotide synthesis and the emergence of DNA in molecular evolution

DNA replication requires monomeric deoxyribonucleotides, which cannot be regarded as primary products of organic syntheses on a primitive earth. However, the present biosynthetic pathway--reductive elimination of the 2'-OH group from ribonucleotides, catalyzed by ribonucleotide reductases and thioredoxins--suggests an early, polyphyletic combination of protein-nucleotide interactions and metal catalysis. That key process had to precede the upcome of RNA-DNA dualism on the way from RNA-protein protocells to true organisms.

Parameters of unbalanced growth and reversible inhibition of deoxyribnucleic acid synthesis in Brevibacterium ammoniagenes ATCC 6872 induced by depletion of Mn2+. Inhibitor studies on the reversibility of deoxyribonucleic acid synthesis

Unbalanced growth induced by depletion of manganese ions was a prerequisite for production of ribonucleotides in a high salt mineral medium with the wildtype strain Brevibacterium ammoniagenes ATCC 6872. The concentration of manganese strictly controlled the overall deoxyribonucleic acid (DNA) synthesis, whereas ribonucleic acid (RNA), protein and cell wall synthesis remained essentially unimpaired in the manganese-lacking cells. The reversibility of inhibition of overall DNA synthesis was shown by enhanced incorporation (up to threefold compared to the cultures supplied with sufficient manganese) of [8-14C] adenine into alkali-stable, trichloroacetic acid-insoluble material after subsequent addition of 10 microM MnCl2 to 15 h-old depleted cultures. The results of inhibitor studies on the restoration of overall DNA synthesis due to subsequent addition of manganese ions to depleted cultures suggest that ribonucleotide reduction is the primary target of the manganese starvation during nucleotide fermentation with Brevibacterium ammoniagenes ATCC 6872.

[Control mechanisms of the ribonucleotide reduction in mammalian tissue (author's transl)]

The ribonucleoside diphosphate reductase (E.C.1.17.4.1) was partially purified from Ehrlich ascites carcinoma and regenerating rat liver. The specific activity of the two enzyme preparations did not vary with regard to the reduction of UDP, ADP, CDP and GDP. Similarly the regulation of the enzyme system by deoxyribonucleoside triphosphate (dNTP) is almost identical. Through the application of deoxyribonucleosides (10(-3) or 2 x 10(-3)M) and measurement of the dNTP content it was found in Ehrlich and Yoshida ascites tumours that these control mechanisms are transmissible to whole cells. dATP inhibits the reduction of all four nucleoside diphosphates. dTTP stimulates the reduction of GDP, dCTP that of UDP and dGTP that of ADP.

[Studies on the thymidine-triphosphate synthesis in malignant tumors. I. Effects of thymidine on deoxyribonucleoside triphosphate pools and deoxyribonucleic acid synthesis (author's transl)]

Measurements of the deoxyribonucleoside triphosphate (dNTP) contents, the [14C] thymidine and deoxyuridine incorporation and the "key enzymes" of the thymidine triphosphate (dTTP) synthesis, thymidine kinase and ribonucleotide reductase, in diploid Ehrlich-ascites carcinoma, in Yoshida sarcoma-ascites cells and to a smaller extent in surgically removed malignant human tumours show 1. A distinctly increased dTTP content compared with the remaining dNTP is not a characteristic of tumour cells generally but a peculiarity of sarcoma and a sign of differentiation of a malignant tumour. 2. With simultaneous linear deoxyribonucleoside incorporation the dTTP content and the mix-proportion of [14C] dTTP to total dTTP in ascites tumour cells in short-term in-vitro incubation (120 min) remain constant. 3. Thymidine addition to the medium leads to a distinct rise of dTTP concentration even at a dosage of 3 X 10(-5) M. 4. The dNTP contents of ascites tumour cells are within the range of the endproduct-inhibiting concentrations of thymidine kinase and ribonucleotide reductase.