Abstract
Intermittent illumination was used to synchronize cells of Chlorella pyrenoidosa. The degree of synchrony of the cultures was evaluated during three consecutive generations of growth in continuous light following the synchronization procedure. The loss in synchrony, as measured by the coefficient of variation, was only 8.53% for the three generations.The uptake and intracellular distribution of [35S]sulfate was followed during synchronous growth. The total cellular-35S (percentage of cellular dry wt.) remained essentially constant throughout cellular development. However, the percentage 35S (percentage of total cellular-35S) in the total cellular protein, cold trichloroacetic acid extract, and sulfolipid fractions of the cells showed dennite periodism.Between the third and fifth hours of cellular development, during a 14-h synchronous growth cycle, the cold trichloroacetic acid-soluble fraction showed a marked increase in percent 35S while the protein fraction showed a decrease of equal magnitude. The decrease in protein-35S was traced to a decrease in the percentage of protein cystine-cysteine-35S. The percentage of protein methionine-35S increased during this period but not enough to compensate for the decrease in protein cystine-cysteine-35S. Thus, the percentage of protein-35S decreased. Between the fifth and seventh hours of cellular maturation, the percentage of 35S in the protein and trichloroacetic acid-soluble fractions returned to their original levels. The protein-sulfur-amino acid composition likewise returned to its original value. The change in the 35S content of the cold trichloroacetic acid-soluble fraction was attributed to a single spot on paper chromatograms which absorbed ultraviolet light and was ninhydrin negative. Acid hydrolysis of the compound(s) in this spot released six or seven ninhydrin-positive spots and two ultraviolet-absorbing spots. After acid hydrolysis, most of the radioactivity was associated with a ninhydrin-positive spot which had the characteristics of cysteic acid.Immediately after the 35S content of the protein and trichloroacetic acid-soluble fractions returned to their original values, nuclear division was initiated. A close relationship between these shifts in sulfur metabolism and the initiation of nuclear division seems possible.
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