PubMed Health⌕ Search

Biomedical subjects

C Basilico

Publications and source records attributed to C Basilico.

At least 127 records · Page 7Linked to original sources
PubMed1979

T-antigen expression in proliferating and non-proliferating simian virus 40-transformed mouse cells.

Previous studies with simian virus 40-transformed mouse 3T3 cells which are temperature sensitive for the expression of the transformed phenotype (ts SV3T3 cells) have shown that T-antigen expression and viral DNA transcription are under cell cycle control. Using these ts SV3T3 cells, we studied the expression of the viral genome under proliferating and non-proliferating conditions, in the presence and absence of inhibitors of macromolecular synthesis and of the tumor promoter phorbol myristate acetate. ts SV3TE cells which are growth arrested at 39 degrees C by low serum concentration or saturation density accumulated in G1 and did not express T-antigen. When these cells were induced to proliferate, at either 32 or 39 degrees C, T-antigen synthesis preceded the entry of the cells into the S-phase and was not coupled to DNA replication. G1-arrested ts SV3T3 cells were induced to synthesize T-antigen by phorbol myristate acetate treatment, but T-antigen alone was not sufficient to induce cellular DNA synthesis. Isoleucine deprivation arrested growth of ts SV3T3 cells, but these cells, as well as normal 3T3, did not accumulate in G1 and continued to express T-antigen. The temperature-sensitive expression of the transformed phenotype in the ts SV3T3 cells does not appear to be due to a lack of transcription of specific regions of the integrated simian virus 40 genome at 39 degrees C.

Animals↗
PubMed1978

Requirements of BHK cells for the exit from different quiescent states.

We have investigated the kinetics of exit from the resting state of BHK cells which had been arrested by isoleucine deprivation, serum starvation, or high temperature in the case of three ts G1 mutants. In addition, we have studied the effect of imposing a secondary deprivation on cells which had been released from one of the above mentioned blocks. The results obtained show that the quiescent states reached by BHK cells following serum or isoleucine deprivation cannot be differentiated on the basis of the exit kinetics from Smith and Martin's probabilistic A-state. Nevertheless, the response of cells to secondary deprivation is different, depending on the nature of the primary arresting condition used, reflecting physiological differences between the different resting states. A model is presented which postulates that cycle transition specific genes require the presence of different proliferative agents for their expression.

Cell Cycle↗
PubMed1978

Analysis of a method for selecting temperature-sensitive mutants of BHK cells.

A procedure for the isolation of temperature sensitive mutants of BHK cells is described. Mutagenized cells were synchronized in G1 by serum starvation at 33 degrees, and, following release, shifted to 37.5 degrees in the presence of FUdR, to kill cells entering the DNA synthetic phase. Eight independently mutagenized series of cells were carried through four cycles of this selection, and surviving cells were tested for ability to grow at 33 degrees and 39.5 degrees after each cycle. The results suggest that such procedure is effective in enriching for non-leaky ts mutants and favors the isolation of mutants which at 39.5 degrees are inhibited in the processes necessary for DNA synthesis and lose viability rapidly. The effectiveness of repeated selection cycles and the general characteristics of the ts mutants isolated by this method were also evaluated.

Cell Line↗
PubMed1978

Premature of chromosome condensation in a ts DNA- mutant of BHK cells.

A temperature-sensitive mutant of BHK, designated ts BN-2, shows a rapid drop in 3H-thymidine incorporation along with accumulation of the cells in the G1 phase of the cycle when asynchronous cultures are shifted from 33.5 degrees C to the nonpermissive temperature of 39.5 degrees C. Synchronized cultures of ts BN-2 cells did not enter DNA synthesis when shifted up in G1. Shift-up of cultures at the beginning of the S phase resulted in an approximately normal rate of DNA synthesis for about 2 hr. The rate of DNA synthesis then quickly declined, and the cells became arrested in mid-S after completion of approximately 0.5 rounds of DNA replication. At the same time, the majority of the cells were observed to lose the nuclear membrane and displayed premature chromosome condensation. These events were followed by the appearance of cells containing several micronuclei and eventual cell disruption and death. The nonpermissive temperature appeared to have no effect on either the elongation of short fragments of DNA or the execution of mitosis after the completion of the S phase under permissive conditions. The ts defect in this mutant may directly limit the initiation of DNA synthesis or alter the regulation of chromatin condensation.

Cell Cycle↗
PubMed1978

Synthesis of H1 histones by BHK cells in G1.

The synthesis of histones and DNA was examined in BHK cells arrested in G1 by isoleucine starvation and in cells progressing into the S phase upon isoleucine refeeding. Approximately 2-3% of the cells were not arrested in G1 and synthesized DNA. The rate of synthesis of DNA and nucleosomal histones observed in cells starved for isoleucine could be accounted for by the presence of these asynchronous cells. Synthesis of H1 histones by cells in G1, however, was 3 times that of the nucleosomal histones and approximately 15% of the rate of H1 histone synthesis in mid-S. Upon entry into S, the histones were synthesized in the same molar ratio in which they are present in chromatin. The possible biological significance of H1 histone synthesis in G1 cells and its implications for the regulatory mechanisms controlling histome synthesis are discussed.

Cell Cycle↗
PubMed1978

Regulation of viral functions in simian virus 40-transformed cells.

To define the relationship between simian virus 40 (SV40)-specific T-antigen and cell growth and to look for regulatory mechanisms that might control T-antigen synthesis in transformed cells, we studied the expression of T-antigen and the viral transcription in SV40-transformed cells that were exponentially growing or arrested in the G1-phase of the cell cycle. We took advantage of the behavior of two lines of SV40-transformed mouse 3T3 cells (ts SV3T3), which, although transformed by wild-type SV40, are temperature sensitive for the expression of the transformed phenotype. At 32 degrees C, ts SV3T3 cells behave like standard transformants, whereas at 39 degrees C, they become arrested in G1 after reaching saturatio n density or under serum starvation. At 32 degrees C or growing at 39 degrees C, ts SV3T3 were 100% T-antigen positive and contained virus-specific mRNA. However, after G1 arrest at 39 degrees C, most of the cells became T-antigen negative. This seems to be caused by a lack of transcription of the integrated viral DNA, since these cells contain no appreciable amounts of SV40-specific RNA. Induction of proliferation in resting, T-antigen-negative ts SV3T3 cultures results in the reappearance of T-antigen a few hours before the cells enter DNA synthesis. These results suggest that transcription of the viral genome and T-antigen expression in SV40-transformed cells is subjected to a cell cycle control.

Animals↗
PubMed1977

Suppression of production of mouse 28S ribosomal RNA in mouse-human hybrids segregating mouse chromosomes.

Mouse-human somatic cell hybrids that lose (segregate) human chromosomes produce only mouse 28S ribosomal RNA even when they retain copies of the human chromosomes that contain the genes for 28S ribosomal RNA. In contrast, mouse-human hybrid cells that segregate mouse chromosomes produce only human 28S ribosomal RNA even when they have retained copies of mouse chromosomes that contain the 28S ribosomal RNA genes.

Animals↗
PubMed1977

State of the viral DNA in rat cells transformed by polyma virus. II. Identification of the cells containing nonintegrated viral DNA and the effect of viral mutations.

F2408 rat cells transformed by polyoma virus contained integrated and nonintegrated viral DNA. The presence of nonintegrated viral DNA is under control of the A early viral function. Polyoma ts-a-transformed rat cells lose the free viral DNA when growth at the nonpermissive temperature (40 degrees C), but they reexpress it 1 to 3 days after they are shifted back to the permissive temperature. In contrast, rat cells transformed by a late viral mutant, ts-8, contain free viral DNA at both permissive and nonpermissive temperatures. Treatment of the transformed rat cells with mitomycin C produces a large increase in the quantity of free viral DNA and some production of infectious virus. Experiments of in situ hybridization, with 3H-labeled polyoma complementary RNA as a probe, show that only a minority (approximately 0.1%) of the transformed cells contain nonintegrated viral DNA at any given time. These results suggest that the presence of free viral DNA in polyoma-transformed rat cells is caused by a spontaneous induction of viral DNA replication, occurring with low but constant probability in the transformed cell population, and that the free viral DNA molecules originate from the integrated ones, probably through a phenomenon of excision and limited replication.

Animals↗
PubMed1976

Processing of ribosomal RNA in a temperature sensitive mutant of BHK cells.

The processing of ribosomal RNA has been studied in a temperature sensitive mutant of the Syrian hamster cell line BHK 21. At 39 degrees C, these cells are unable to synthesize 28S RNA, and 60S ribosomal subunits, while 18S RNA, and 40S subunits are produced at both temperatures. At 39 degrees C the 45S RNA precursor is transcribed and processed as in wild type cells. The processing of the RNA precursors becomes defective after the cleavage of the 41S RNA, and the separation of the 18S and 28S RNAs sequences in two different RNA molecules. The 36S RNA precursor, which is always present in very small quantity in the nucleoli of wild type cells and of the mutant at 33 degrees C, is found in very large amounts in the mutant at 39 degrees C. The 36S RNA can be, however, slowly processed to 32S RNA. The 32S RNA cannot be processed at 39 degrees C, and it is degraded soon after its formation. Only a small proportion accumulates in the nucleoli. The 32S RNA synthesized at 39 degrees C cannot be processed to 28S RNA upon shift to the permissive temperature, even when the processing of the newly synthesized rRNA has returned to normal. The data suggest that the 36S and 32S RNAs are contained in aberrant ribonucleoprotein particles, leading to a defective processing of the particles as a whole.

Cell Line↗
PubMed1976

Regulation of viral transciption and tumor antigen expression in cells transformed by simian virus 40.

We have studied the expression of simian virus 40 (SV40) specific tumor antigen (T-antigen) and viral RNA in SV40-transformed mouse 3T3 cells that are temperature-sensitive for the expression of the transformed phenotype (ts SV3T3). Although transformed by wild-type SV40, ts SV3T3 cells at 32 degrees behave like standard transformants, while at 39 degrees they became arrested in G1 after reaching saturation density or under conditions of serum starvation. ts SV3T3 cells at 32 degrees or exponentially growing at 39 degrees are uniformly T-antigen positive. However, after G1 arrest at 39 degrees the majority of the cells becomes T-antigen negative. Induction of proliferation in the resting cultures results in the reappearance of T-antigen in most of the cells, concomitant with the induction of DNA synthesis. The reason for the disappearance of T-antigen from ts SV3T3 cells arrested in G1 seems to reside in a transcriptional control operating on the integrated viral DNA, since these cells contain no appreciable amounts of SV40 specific RNA. Viral RNA can be easily detected in cells growint at 32 degrees or at 39 degrees. The results suggest that transcription of the viral genome in SV40-transformed cells is cell-cycle-dependent.

Antigens, Viral↗
PubMed1976

State of the viral DNA in rat cells transformed by polyoma virus. I. Virus rescue and the presence of nonintergrated viral DNA molecules.

The interaction of polyoma virus with a continuous line of rat cells was studied. Infection of these cells with polyoma did not cause virus multiplication but induced transformation. Transformed cells did not produce infectious virus, but in all clones tested virus was rescuable upon fusion with permissive mouse cells. Transformed rat cells contained, in addition to integrated viral genomes, 20 to 50 copies of nonintegrated viral DNA equivalents per cell (average). "Free" viral DNA molecules were also found in cells transformed by the ts-a and ts-8 polyoma mutants and kept at 33 C. This was not due to a virus carrier state, since the number of nonintegrated viral DNA molecules was found to be unchanged when cells were grown in the presence of antipolyoma serum. Recloning of the transformed cell lines produced subclones, which also contained free viral DNA. Most of these molecules were supercoiled and were found in the muclei of the transformed cells. The nonintegrated viral DNA is infectious. Its specifici infectivity is, however, about 100-fold lower than that of polyoma DNA extracted from productively infected cells, suggesting that these molecules contain a large proportion of defectives.

Animals↗