Utilization of deoxyribonucleosides by virulent and avirulent pneumococci

Abstract

The utilization of deoxyribonucleosides was studied in virulent and avirulent pneumococci. It was found that virulent pneumococci incorporated five [ 14C]deoxyribonucleosides into DNA at a greater rate and extent than avirulent pneumococci. However, when the polysaccharide capsule surrounding virulent cells was removed by a hydrolytic enzyme, incorporation was reduced considerably. 1. 1. The rate of uptake of [ 14C]deoxyribonucleosides was greater in virulent than avirulent pneumococci, but the differences were greater for pyrimidine deoxyribonucleosides than for purine deoxyribonucleosides. 2. 2. Almost all of the radioactivity found in DNA of virulent and avirulent cells after incorporation of deoxycytidine, deoxyuridine and thymidine was localized in the thymine moiety. No significant radioactivity was detected in the cytosine moiety. The pathway of deoxycytidine, deoxyuridine and thymidine utilization, for which all of the intermediates were detected in studies with cell-free extracts, was as follows: ▪ No deoxycytidine kinase or dCMP deaminase were detected. More radioactivity was localized in the thymine moiety of DNA when deoxycytidine was used as the substrate than when thymidine was employed, despite the same initial specific activity. 3. 3. Radioactivity from deoxyadenosine, deoxyguanosine and deoxyinosine that was incorporated into DNA of virulent cells was found both in the adenine and guanine moieties. In avirulent cells, only the radioactivity from deoxyguanosine was found in these two purine moieties, while label from deoxyadenosine or deoxyinosine was found exclusively in the adenine moiety of avirulent DNA. Not all of the intermediates were found in cell-free extracts for any single pathway involving purine deoxyribonucleosides but by combining the results obtained with the three of them, an almost complete picture of metabolism was revealed. 4. 4. Whereas virulent cells were capable of converting deoxyinosine (or dIMP) to deoxyxanthosine or dXMP, and the latter to deoxyguanosine (or dGMP), avirulent cells could not carry out these reactions. 5. 5. Almost 70–80% of the radioactivity found in the nucleic acid (DNA and RNA) fraction after incorporation of purine deoxyribonucleosides was removed by alkaline hydrolysis and subsequent acid precipitation of the DNA. This was not the case with pyrimidine deoxyribonucleosides.