Study on conformational states of Escherichia coli tRNAPhe in solution by a modulation-free ESR-spectrometer.

A modulation free Electron Spin Resonance spectrometer was used for the registration of spectral absorption lines of a spin-labeled Escherichia coli phenylalanine tRNA in solution with low (less than 0.1%) line shape distortion. The analysis of line shape of two different spin-labels introduced into position 8 revealed that phenylalanine tRNA in solution exists as a mixture of two conformers, the equilibria between conformers being dependent on pH, concentration of magnesium and functional state of tRNA (deacylated, aminoacylated or peptidylated). There are no overall structural rearrangements upon aminoacylation or peptidylation of tRNA. The observed small changes of spectral line shape can be assigned to shifts in conformational equilibria.

[DNA replication in HeLa cells after gamma irradiation. II. The periodicity of DNA replication in the process of giant cell formation]. / Replikatsiia DNK v kletkakh HeLa posle gamma-oblucheniia. II. Periodichnost' replikatsii DNK v protsesse obrazovaniia gigantskikh kletok.

HeLa G-63 cells irradiated by 5 krads of 6 degrees Co gamma-rays 1.5--2 hours after mitosis (G1-phase) were incubated in growth medium during 9 days. The number of proliferating and eliminated cells, the content of DNA per cell nucleus, and kinetics of the labeled cell fraction upon 3H-TdR continuous incubation were studied. It has been concluded that: during the giant cell growth the nuclear DNA is replicated periodically without following mitoses; 90% of cells are involved in such a cycle; the duration of the period of DNA synthesis in giant cells is the same as that in non-irradiated cells; the duration of the gap between two successive S-periods in giant cells is 2--6 times as long as the sum of G2+M+G1 periods in the normal non-irradiated cells.

[Curve of labeled mitoses with different states of cell proliferation kinetics. V. Effect of the circadian rhythm of cell proliferation on the shape of the labeled mitosis curve]. / Krivaia mechenykh mitozov pri razlichnykh sostoianiiakh kinetiki kletochnoi proliferatsii. V. Vliianie sutochnogo ritma kletochnoi proliferatsii na formu krivoi mechenykh mitozov.

The influence of the diurnal rhythm in cell proliferation processes on the behavior of labeled mitoses curve (LMC) is under investigation. The mathematical model predicts that LMC observed following 3H-thymidine administration to experimental animals at different day-times can differ greatly even when temporal parameters of the mitotic cycle remain unchanged. The analysis of the experimental data on the hamster's cheek pouch epithelium suggests that diurnal differences in LMCs cannot be explained in certain cases by the influences of transient processes in cell kinetics only, and therefore the existence of a slight trend in prolongation of periods of the mitotic cycle cannot be completely excluded.