The electrostatic role of the Zn-Cys2His2 complex in binding of operator DNA with transcription factors: mouse EGR-1 from the Cys2His2 family

The mouse factor Zif268, known also as early growth response protein EGR-1, is a classical representative for the Cys2His2 transcription factor family. It is required for binding the RNA polymerase with operator dsDNA to initialize the transcription process. We have shown that only in this family of total six Zn-finger protein families the Zn complex plays a significant role in the protein-DNA binding. Electrostatic feature of this complex in the binding of factor Zif268 from Mus musculus with operator DNA has been considered. The factor consists of three similar Zn-finger units which bind with triplets of coding DNA. Essential contacts of the factor with the DNA phosphates are formed by three conservative His residues, one in each finger. We describe here the results of calculations of the electrostatic potentials for the Zn-Cys2His2 complex, Zn-finger unit 1, and the whole transcription factor. The potential of Zif268 has a positive area on the factor surface, and it corresponds exactly to the binding sites of each of Zn-finger units. The main part of these areas is determined by conservative His residues, which form contacts with the DNA phosphate groups. Our result shows that the electrostatic positive potential of this histidine residue is enhanced due to the Zn complex. The other contacts of the Zn-finger with DNA are related to nucleotide bases, and they are responsible for the sequence-specific binding with DNA. This result may be extended to all other members of the Cys2His2 transcription factor family.

[Chronobiological analysis of thyroxine action on cell proliferation in the hypotetraploid strain of Ehrlich's ascitic tumor]

Experiments on white random-bred male mice were made to study the effect of L-thyroxine on cell proliferation of the hypotetraploid strain of Ehrlich's ascites carcinoma. It was shown that prolonged thyroxine administration (during 6 days of carcinoma growth) lead to synchronization of cell proliferation and the maximum values of the mitotic index was found 3 hours earlier then in the control experiments. At the same time thyroxine did not exert any noticeable effect on the average daily magnitudes of the number of DNA-synthesizing cells and did not change the pattern of modulations in the radioactive index. The changes in the mitotic index and radioactive index were asynchronous in control and experimental animals. Analogous results were found for hyperdiploid strain of Ehrlich's ascites tumor. Ploidy of cells did not influence the tipe rhythms of the cell proliferation and its reaction on the action of thyroxine.

[Temporal organization of the cellular reproduction processes in Ehrlich ascites tumor]

Experiments were made on randombred male mice weighing 18-20 g maintained on the light regimen L:D-12:12. The animals were given a single injection i. p. of a hypotetraploid subline of Ehrlich's ascites tumor. Fluctuations in the number of mitotic cells and no changes in the number of DNA-synthesizing cells were established. This indicates that synchronization of tumor cells under these conditions occurs mainly in the G2 phase. The results of the present experiments confirm those obtained during previous investigations, showing the lack of the dependence of the cell proliferation pattern on the cell ploidy. The disturbance of the chronobiological organization of tumor cell proliferation that is manifested by upset synchronization of cell division and DNA synthesis is common to all Ehrlich's ascites tumor sublines under study.

[Chronobiological patterns of the action of thyroxine on cell reproduction in Ehrlich ascites tumor]

Experiments on white random-bred male mice were made to study the effect of L-thyroxine on cell proliferation of the hyperdiploid strain of Ehrlich's ascites carcinoma. It was shown that upon prolonged thyroxine administration (during 6 days of carcinoma growth) the action of the latter appeared dose-dependent. The doses used were 1, 10, 30, 60, 100 micrograms/100 g bw. Circadian rhythms were noted in the sensitivity of the dividing cells to the hormone applied in a dose of 10 micrograms/100 g bw as was a certain decrease in the total level of the mitotic activity. At the same time thyroxine did not exert any noticeable effect on the average daily magnitudes of the number of DNA-synthesizing cells and did not change the pattern of modulations in the radioactive index. The changes in the mitotic index and radioactive index were asynchronous in control and experimental animals.

[Circadian changes in mitotic activity and DNA synthesis in hyperdiploid and hypotetraploid strains of Ehrlich's ascites tumor]

Noninbred male white mice received intraperitoneal injections of hyperdiploid or hypotetraploid strain of Ehrlich's ascites tumour. Fluctuations in the mitotic index were found both in the hypotetraploid strain at natural photoregime and in the hyperdiploid one with the period L:D-12:12 by the 4th, 5th and 6th days from the onset of tumour growth. The mitotic index reached the maximum values in the morning and in the day time. Circadian fluctuations of the mitotic and radioactive indices were not synchronous in different strains and under varying photoregimes. The data obtained point to significant differences in time organization of cell proliferation in Ehrlich's ascites tumour as compared to normal tissues.