DNA and histone synthesis in mouse cells which exhibit temperature-sensitive DNA synthesis

Temperature-sensitive Chinese hamster fibroblast mutant with a defect in RNA metabolism

We describe a new temperature-sensitive mutant of Chinese hamster cell fibroblasts. After a shift to the nonpermissive temperature of 40.5 degrees C, the rates of DNA, RNA, and protein synthesis declined rapidly (to < or = 50% within 12 h) and the progression of unsynchronized cells through the cell cycle was affected. We believe that DNA synthesis came to a halt after a short time, because cells no longer entered the S phase. The decrease in protein synthesis at 40.5 degrees C was shown to be a consequence of a decrease in the number of polysomes, whereas free 80S ribosomes accumulated. We concluded that the components of the protein biosynthetic machinery were intact (ribosomes and soluble factors), but synthesis was limited by a shortage of mRNA. The decline in mRNA production had a significant effect on the synthesis of proteins (e.g., heat shock proteins) translated from short-lived messages. We observed that both polyadenylated and nonpolyadenylated RNA syntheses declined at 40.5 degrees C, whereas the synthesis of small RNAs (4 to 5S) was less reduced. The argument is made that the temperature-sensitive phenotype is the result of a defect affecting mRNA synthesis.

Gene on short arm of human X chromosome complements murine tsA1S9 DNA synthesis mutation

We have created somatic cell hybrids between the temperature-sensitive mouse cell line tsA1S9 and human cell lines in order to localize the human gene (A1S9T) complementing the cell cycle defect of the murine line. Segregation of the human X chromosome is completely concordant with growth at the nonpermissive temperature. Hybrids retaining the X chromosome are temperature-resistant, whereas those without a human X are temperature-sensitive. Further hybrids made using human cell lines with X-autosome translocations indicate that the A1S9T gene is located on the short arm of the human X chromosome.

Cell cycle arrest of heat-inactivated ts 2 BALB/c-3T3 mouse fibroblasts, temperature-sensitive for DNA synthesis

The ts 2 derivative of BALB/c-3T3 mouse fibroblasts is a cell division cycle (cdc) mutant. Upon expression of the heat-sensitive defect, ts 2 cells arrest late in G1 at, or very near the G1/S traverse. This conclusion derives from three kinds of experiment. In the first the cells were brought to different stages of the cell cycle by physiological manipulation, or with specific anti-metabolites. They were then released from the resulting blocks, and their subsequent cell-cycle progression, at the permissive- and non-permissive temperature (npt), was followed. The second experiment was an execution point analysis. In the third, premature chromosome condensation was performed between metaphase HeLa cells and temperature-blocked ts 2 cells. The resulting prematurely-condensed chromosomes were largely of the morphotype of very late G1 cells. The ts 2 cells are prevented from expressing their defect by temporary incubation at 38.5 degrees C in the G0, non-cycling state and by prior arrest in early S phase, imposed by hydroxyurea treatment. Such prevention is not allowed ts 2 cells incubated at the npt in the absence of isoleucine, a procedure which brings cells to mid-G1 arrest.

Exchange of histones H1, H2A, and H2B in vivo

We have asked whether histones synthesized in the absence of DNA synthesis can exchange into nucleosomal structures. DNA synthesis was inhibited by incubating hepatoma tissue culture cells in medium containing 5.0 mM hydroxyurea for 40 min. During the final 20 min, the cells were pulsed with [3H]lysine to radiolabel the histones (all five histones are substantially labeled under these conditions). By two electrophoretic techniques, we demonstrate that histones H1, H2A, and H2B synthesized in the presence of hydroxyurea do not merely associate with the surface of the chromatin but instead exchange with preexisting histones so that for the latter two histones there is incorporation into nucleosome structures. On the other hand, H3 and H4 synthesized during this same time period appear to be only weakly bound, if at all, to chromatin. These two histones have been isolated from postnuclear washes and purified. Some possible implications of in vivo exchange are discussed.

A gene function required for cell cycle progression during the G1 portion of the cell cycle and for maintenance of macronuclear DNA synthesis in Paramecium tetraurelia

The ccl mutation in Paramecium tetraurelia reversibly and rapidly blocks cell cycle progression and DNA synthesis at the restrictive temperature. Progression through the cell cycle is blocked during both the G1 and S portions of the cell cycle, while at the restrictive temperature there is neither residual cell cycle progression nor induction of excess delay of subsequent cell cycle events. DNA synthesis activity is reduced to 50% of the normal level in about 5 min and is completely blocked at 30 min after a shift to restrictive temperature. On return to permissive conditions, DNA synthesis is reactivated with similar kinetics.

Coexpression of two thermosensitive defects in a Chinese hamster cell line. Manifestation of a G1 block associated with a mitosis defect

Asynchronous cultures of ts12, an anchorage-dependent derivative of the thermosensitive Chinese hamster cell line ts111, show a rapid drop in [3H]thymidine incorporation with accumulation of the cells in the G1 and in the G2 phases of the cycle, when shifted from 34.5 to 39.4 degrees C. Shift-up experiments carried out after either isoleucine deprivation or synchronization at 39.4 degrees C, locate the execution point of a ts function in late G1 (2.5-3 h before S). However, stimulation of proliferation of a high density-arrested population allows a fraction of the cells to enter S. In addition to the G1 ts defect, ts12 expresses a slight cytokinesis defect at 39.4 degrees C (8-15% binucleate cells). The results suggest that altered processes are taking place at a post-metaphasic stage during the first hours after the shift-up. When populations are synchronized by a thymidine block and released at 39.4 degrees C, multinucleate cells in addition to binucleate cells are observed. Part of these multinucleate cells result from abnormal karyokinesis without inhibition of cytokinesis. Evidence is presented suggesting that excess thymidine allows the re-expression of the multinucleation phenotype of ts111.

In vitro exchange of nucleosomal histones H2a and H2b

We have asked whether exogenous, radiolabeled histones can exchange with nucleosomal histones in an in vitro system. Using two different electrophoretic techniques, we were able to separate the histones contained in nucleosomes from those histones which were simply bound to the surface of the chromatin. Fluorography was used to determine which of the exogenous histones exchange with the nucleosomal histones. We observed substantial exchange of histones H1, H2a, and H2b when the chromatin and exogenous histones were incubated under approximately physiological conditions. We have also observed a small amount of exchange of H2a and H2b, as well as a substantial exchange of H1, from one chromatin fragment to another. Other conditions affecting the exchange of histones H2a and H2b are also reported.

Temperature-Sensitive Chinese Hamster Fibroblast Mutant with a Defect in RNA Metabolism

We describe a new temperature-sensitive mutant of Chinese hamster cell fibroblasts. After a shift to the nonpermissive temperature of 40.5°C, the rates of DNA, RNA, and protein synthesis declined rapidly (to ≤50% within 12 h) and the progression of unsynchronized cells through the cell cycle was affected. We believe that DNA synthesis came to a halt after a short time, because cells no longer entered the S phase. The decrease in protein synthesis at 40.5°C was shown to be a consequence of a decrease in the number of polysomes, whereas free 80S ribosomes accumulated. We concluded that the components of the protein biosynthetic machinery were intact (ribosomes and soluble factors), but synthesis was limited by a shortage of mRNA. The decline in mRNA production had a significant effect on the synthesis of proteins (e.g., heat shock proteins) translated from short-lived messages. We observed that both polyadenylated and nonpolyadenylated RNA syntheses declined at 40.5°C, whereas the synthesis of small RNAs (4 to 5S) was less reduced. The argument is made that the temperature-sensitive phenotype is the result of a defect affecting mRNA synthesis.

A temperature-sensitive DNA synthesis mutant isolated from the Chinese hamster ovary cell line

A temperature-sensitive DNA synthesis mutant, tsC8, was isolated from mutagenized Chinese hamster ovary cells by the fluorodeoxyuridine suicide technique. The tsC8 cells showed inhibition of DNA synthesis at the nonpermissive temperature (NPT) with little effect on initial levels of RNA and protein synthesis. Temperature-arrested tsC8 cells had G1 or S DNA content and the temperature-sensitive (ts) period of the tsC8 cell cycle was the interval between the G1/S border and the middle of the S period. The tsC8 cells were unable to enter the S phase when exposed to the NPT during the G1 period of the cell cycle. When S phase tsC8 cells were shifted to the NPT, they incorporated [3H]thymidine at rates similar to the parental cell type for only 2 h, indicating a ts defect in DNA synthesis. The tsC8 mutation is expressed in a recessive manner and is in a gene distinct from those affected in other DNA synthesis mammalian cell mutants.