Abstract

By feeding radioisotopic precursors of RNA ([5-3H]uracil and [5-3H]uridine) to cells of Chlorella ellipsoidea at various stages in the cell cycle effected by autotrophic synchronous culture, we examined synthetic rates of the chloroplast and the cytoplasmic ribosomal ribonucleic acids (chl-rRNA and cyt-rRNA, respectively). The net incorporation of the precursors into chl-rRNA was higher than that into cyt-rRNA in the early stages of the cell cycle, and vice versa in the late stages. The specific activity of chl-rRNA was extremely high, and this phenomenon was likely to be intrinsic to small cells at the start of the cell cycle under autotrophic conditions, namely, cell-cycle stagespecific. We conclude that algal cells grown autotrophically synthesize chl-rRNA at a distinctly higher rate than cyt-rRNA in the early stages of the cell cycle.

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