Role of silicon in diatom metabolism III. The effects of silicic acid on DNA polymerase, TMP kinase and DNA synthesis in Cylindrotheca fusiformis

Abstract

1. An enzyme with the characteristics of a DNA polymerase was found in cell-free extracts of the diatom Cylindrotheca fusiformis. The activity occurring in the 40 000 × g supernatant fraction required denatured DNA, Mg 2+, the four deoxyribonucleoside 5′-triphosphates, and was stimulated 50–150 % by 2 mM ATP. It was found to be inhibited by p- chloromercuribenzoate , N- ethylmaleimide and the 5′-triphosphate of 1β- d-arabinofuranosylcytosine. 2. DNA polymerase specific activity fluctuates during the light-dark synchronized division cycle of C. fusiformis. The fluctuation is not the result of cyclic influence of effectors, deoxyribonuclease activity, limitation of precursors, or change in enzyme location. 3. There was 64 % less DNA polymerase activity in sychronized cells deprived of silicic acid than in control cells grown in the presence of silicic acid. An immediate and specific increase in DNA polymerase and TMP kinase specific activities and in DNA net synthesis occurred in silicic acid-deprived cells upon the addition of silicic acid to the culture medium; however, stimulation of these enzymes by silicic acid could not be demonstrated in vitro. 4. Stimulation of the enzyme activity during the division cycle of synchronized cells previously starved for 24 h was sensitive to inhibitors of protein synthesis but not to actinomycin D. It appears, therefore, that silicic acid may increase the activity of TMP kinase and DNA polymerase by acting at some post-transcriptional site. 5. After stimulation by silicic acid, continued DNA net synthesis was dependent at all times on concomitant protein synthesis.