The sexual activity of aged Paramecium that underwent about 700 fissions after conjugation was remarkably decreased. Microinjection of a soluble cytoplasmic fraction prepared from young immature cells (about 20 fissions after conjugation) restored the sexual activity of the aged cells to that of presenescent mature cells. The factor responsible for the recovery of the sexual activity was shown to have the same characteristics as immaturin in terms of molecular size and net charge. Rejuvenation of sexual activity lasted up to 20 fissions after microinjection. However, cell division rate or fertility after self-conjugation was not increased by the microinjection. Our finding indicates that the decrease in sexual activity caused by aging was not due to defects in the genes for mating substances but to a decrease in the activity of a soluble protein responsible for the expression of mating activity. This is the first description of the restoration of a senescent defect by microinjection of young cytoplasmic components into Paramecium.

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