The uptake of bases and their incorporation into RNA during the cell cycle of Schizosaccharomyces pombe in normal growth and after a step-down

Abstract

The uptake and incorporation of adenine and uracil during the cell cycle of the fission yeast Schizosaccharomyces pombe in normal growth was measured by grain counting in autoradiographs and by bulk counting in synchronous cultures. The following cellular properties increased steadily and in proportion through the cycle: initial rate of uptake, rate of incorporation, size of the preexisting precursor pool and, with adenine, the size of the expanded pool. Similar patterns of incorporation and uptake were shown for guanine, cytosine, guanosine and cytidine. Both ribosomal RNA and rapidly labelled nuclear RNA were synthesised at an increasing rate through the cycle and followed an approximately exponential curve of synthesis. There was no indication of changes in the base ratios. The incorporation rates did not appear to be distorted by changes in uptake rate or pool size. The uptake and incorporation into step-down RNA was followed by autoradiography. It differed from the other two RNA's in showing a marked shift in the ratio of adenine/uracil incorporated at the end of the cycle. This is probably due to a change in the base composition of the RNA and not to changes in uptake rate or pool size. About half the step-down RNA was found in the cytoplasm after a short pulse label.