Reactivation of stationary Tetrahymena. A contribution to the question of G0 state

Inactivation of a macronuclear intra-S-phase checkpoint in Tetrahymena thermophila with caffeine affects the integrity of the micronuclear genome

Aphidicolin (APH), an inhibitor of DNA polymerase α, arrested cell divisions in Tetrahymena thermophila. Surprisingly, low concentrations of APH induced an increase of macronuclear DNA content and cell size in non-dividing cells. In spite of the cell size increase, most proliferation of basal bodies, ciliogenesis and development of new oral primordia were prevented by the APH treatment. The division arrest induced by APH was partly overridden by caffeine (CAF) treatment, which caused the fragmentation ("pulverization") of the chromosomes in G2 micronuclei. Somatic progeny of dividers with pulverized micronuclei (APH+CAF strains) contained aneuploid and amicronucleate cells. The amicronucleate cells, after losing their oral structures and most of their cilia, and undergoing progressive disorganization of cortical structures, assumed an irregular shape ("crinkled") and were nonviable. "Crinkled" cells were not formed after APH + CAF treatment of the amicronuclear BI3840 strain, which contains some mic-specific sequences in its macronucleus. Most of the APH +CAF strains had a typical "*"- like conjugation phenotype: they did not produce pronuclei, but received them unilaterally from their mates and retained old macronuclei. However, 4 among 100 APH+CAF clones induced arrest at meiotic metaphase I in their wt mates. It is likely that the origin of such clones was enhanced by chromosome pulverization.

A temperature-compensated ultradian clock of Tetrahymena: oscillations in respiratory activity and cell division

Both a circadian clock and an ultradian clock (period 4-5 h) have previously been described for the ciliated protozoon Tetrahymena. The present communication demonstrates the existence of yet another cellular clock: an ultradian rhythm with a period of about 30 min. The period was found to be well temperature-compensated over the range studied, i.e., between 19 degrees C and 33 degrees C. Ultradian rhythmicity was initiated by dilution of stationary-phase cultures, which were kept previously in a light-dark cycle, into fresh medium. LD treatment during stationary phase was an absolute requirement, since cultures kept in either LL or DD did not produce the ultradian rhythmicity after refeeding. The clock exerts control over respiration; the observed oscillation in oxygen uptake is just a hand of the clock: after a limitation of oxygen supply had ended, the rhythm resumed with the same phase and period as that in control cultures. The clock exerts temporal control also over cell division; in the refed culture cell division resumed with an oscillation in the number of dividing organisms. The period of this oscillation corresponded to that of the rhythm in respiratory activity, indicating that the same ultradian clock may exert control over different cellular functions. Analysis of a second Tetrahymena strain indicates that period length of the ultradian clock is a strain-specific characteristic.

The timing of initiation of macronuclear DNA synthesis is set during the preceding cell cycle in Paramecium tetraurelia. Analysis of the effects of abrupt changes in nutrient level

In many eukaryotic organisms, initiation of DNA synthesis is associated with a major control point within the cell cycle and reflects the commitment of the cell to the DNA replication-division portion of the cell cycle. In Paramecium, the timing of DNA synthesis initiation is established prior to fission during the preceding cell cycle. DNA synthesis normally starts at 0.25 in the cell cycle. When dividing cells are subjected to abrupt nutrient shift-up by transfer from a chemostat culture to medium with excess food, or shift-down from a well-fed culture to exhausted medium. DNA synthesis initiation in the post-shift cell cycle occurs at 0.25 of the parental cell cycle and not at either 0.25 in the post-shift cell cycle or at 0.25 in the equilibrium cell cycle produced under the post-shift conditions. The long delay prior to initiation of DNA synthesis following nutritional shift-up is not a consequence of continued slow growth because the rate of protein synthesis increases rapidly to the normal level after shift-up. Analysis of the relation between increase in cell mass and initiation of DNA synthesis following nutritional shifts indicates that increase in cell mass, per se, is neither a necessary nor a sufficient condition for initiation of DNA synthesis, in spite of the strong association between accumulation of cell mass and initiation of DNA synthesis in cells growing under steady-state conditions.

Cell cycle controls in higher eukaryotic cells: resting state or a prolonged G1 period?

We express the viewpoint that control over cell growth in higher eukaryotes is achieved predominantly by regular transition of cells from proliferation to rest and vice versa as a result of coordinated interrelationship between intracellular growth inhibitors and extracellular growth factors. The resting state is considered as a special physiological state of a cell where the prereplicative reactions necessary for the onset of DNA synthesis are inhibited. Cells pass into a resting state at each successive cell cycle, with regard to the next cycle, once the threshold level of growth inhibitors has been attained. Cellular rest may thus initiate and proceed in parallel with conventional periods of the cell cycle but in a hidden way. Its termination strictly depends on the appropriate concentration of extracellular growth factors. In the absence of growth factors cells, after completing mitosis, pass into an overt state of rest metabolically different from any period of the cell cycle including G1.

The growth inhibition in Friend erythroleukemia cells induces cycling of preexisting nonhistone proteins

The present investigation performed on Friend erythroleukemia cells provides evidence that the cellular reprogramming associated with transition of the cells from the growing to the resting state is accompanied by an intranuclear cycling of preexisting proteins migrating from the cytoplasm. The model system developed for this study affords an opportunity to follow these events in a conveniently short period. The flow microfluorometric analysis revealed that, with respect to DNA content, the quiescent nuclei exhibit distribution in G1, S and G2 phases, as do the nuclei of the control, exponentially growing cells. It was found further that the induction of quiescence arouses two waves of migration of preexisting cytoplasmic proteins toward the nucleus: an early one, during the transition to quiescence clearly showing density dependence, and a late one, in the established resting state. While the early-cycled proteins are undetectable in mass and once they have reached the nucleus have a short life time and, thus, could be considered as regulatory molecules, the late-cycled proteins accumulate within the nucleus and are associated most probably with structural reorganization of the resting nuclei; moreover, the late-cycled nonhistone proteins were found localized, at least partly, in the nuclear protein matrix structure. Control experiments confirmed that the early and late-cycled proteins have been synthesized in the cytoplasm in the proliferative state but are cycled intranuclearly only if the resting state has been induced. When finally the resting cells were stimulated to proliferate, the accumulated presynthesized and newly synthesized nonhistone proteins undergo a rapid degradation which suggests that the new cellular programme may require a complete elimination of the 'resting' nonhistone proteins.