Polyoma and cell DNA synthesis in mouse L cells temperature sensitive for the replication of cell DNA

Molecular cloning of human A1S9 locus: an X-linked gene essential for progression through S phase of the cell cycle

The temperature-sensitive (ts) A1S9 mouse L-cell mutant is defective in an X-linked gene essential for the progression of cells through the S phase of the cell duplication cycle. We recently reported the complementation of the ts A1S9 cell defect with total human DNA and the isolation of independent temperature-resistant transformants that retained a common set of human specific Alu-containing fragments. Here we describe the molecular cloning of these human DNA sequences from one of the secondary transformants. ST-1-0. A genomic library prepared from ST-1-0 was screened with a total human DNA probe, and two recombinant bacteriophages carrying overlapping segments were isolated. The cloned region was extended in both directions using a human X-chromosome specific library. In total, a human region spanning 42 kb in length, and containing all the Alu-specific DNA sequences found in ST-1-0, was isolated in five overlapping recombinant phages. The A1S9 gene appeared to be larger than the DNA recovered in individual phage isolates, as was assessed by transfection experiments. A single-copy probe derived from the phage DNA was shown to be conserved in independent primary, secondary, and tertiary transformants of ts A1S9 cells and mapped to the X chromosome by molecular hybridization. Northern blot hybridization of this probe with poly(A)+ mRNA derived from ST-1-0 cells identified a transcript of about 3.6 kb.

Identification of temperature-sensitive DNA- mutants of Chinese hamster cells affected in cellular and viral DNA synthesis

We described a strategy which facilitates the identification of cell mutants which are restricted in DNA synthesis in a temperature-dependent manner. A collection of over 200 cell mutants temperature-sensitive for growth was isolated in established Chinese hamster cell lines (CHO and V79) by a variety of selective and nonselective techniques. Approximately 10% of these mutants were identified as ts DNA- based on differential inhibition of macromolecular synthesis at the restrictive temperature (39 degrees C) as assessed by incorporation of [3H]thymidine and [35S]methionine. Nine such mutants, selected for further study, demonstrated rapid shutoff of DNA replication at 39 degrees C. Infections with two classes of DNA viruses extensively dependent on host-cell functions for their replication were used to distinguish defects in DNA synthesis itself from those predominantly affecting other aspects of DNA replication. All cell mutants supported human adenovirus type 2 (Ad2) and mouse polyomavirus DNA synthesis at the permissive temperature. Five of the nine mutants (JB3-B, JB3-O, JB7-K, JB8-D, and JB11-J) restricted polyomavirus DNA replication upon transfection with viral sequences at 33 degrees C and subsequent shift to 39 degrees C either before or after the onset of viral DNA synthesis. Only one of these mutants (JB3-B) also restricted Ad2 DNA synthesis after virion infection under comparable conditions. No mutant was both restrictive for Ad2 and permissive for polyomavirus DNA synthesis at 39 degrees C. The differential effect of these cell mutants on viral DNA synthesis is expected to assist subsequent definition of the biochemical defect responsible.

Identification of temperature-sensitive DNA- mutants of Chinese hamster cells affected in cellular and viral DNA synthesis

We described a strategy which facilitates the identification of cell mutants which are restricted in DNA synthesis in a temperature-dependent manner. A collection of over 200 cell mutants temperature-sensitive for growth was isolated in established Chinese hamster cell lines (CHO and V79) by a variety of selective and nonselective techniques. Approximately 10% of these mutants were identified as ts DNA- based on differential inhibition of macromolecular synthesis at the restrictive temperature (39 degrees C) as assessed by incorporation of [3H]thymidine and [35S]methionine. Nine such mutants, selected for further study, demonstrated rapid shutoff of DNA replication at 39 degrees C. Infections with two classes of DNA viruses extensively dependent on host-cell functions for their replication were used to distinguish defects in DNA synthesis itself from those predominantly affecting other aspects of DNA replication. All cell mutants supported human adenovirus type 2 (Ad2) and mouse polyomavirus DNA synthesis at the permissive temperature. Five of the nine mutants (JB3-B, JB3-O, JB7-K, JB8-D, and JB11-J) restricted polyomavirus DNA replication upon transfection with viral sequences at 33 degrees C and subsequent shift to 39 degrees C either before or after the onset of viral DNA synthesis. Only one of these mutants (JB3-B) also restricted Ad2 DNA synthesis after virion infection under comparable conditions. No mutant was both restrictive for Ad2 and permissive for polyomavirus DNA synthesis at 39 degrees C. The differential effect of these cell mutants on viral DNA synthesis is expected to assist subsequent definition of the biochemical defect responsible.

ts A1S9 locus in mouse L cells may encode a novobiocin binding protein that is required for DNA topoisomerase II activity

Nuclear novobiocin binding proteins (NBPs) from a set of mouse L cells have been extensively purified by affinity chromatography on novobiocin-Sepharose columns. The NBPs, specifically eluted with 100 micrograms of novobiocin per ml, exhibited equivalent DNA topoisomerase activities (measured as ATP-dependent relaxation or catenation of phi X174 replicative-form I DNA substrate) when extracted from equal numbers of wild-type (WT-4) mouse L cells growing logarithmically at 34 degrees C or at 38.5 degrees C, from ts A1S9 cells similarly cultivated at the low, permissive temperature or from revertant ts+ AR cells in exponential growth at either temperature. The NBPs isolated from similar numbers of ts A1S9 cells grown to midlogarithmic phase and then incubated for 24 hr at 38.5 degrees C (the nonpermissive temperature) showed no topoisomerase II activity. Preliminary NaDodSO4/polyacrylamide gel electrophoretic analysis of enzymatically active material revealed that the NBPs of WT-4 and ts+ AR cells grown at 34 degrees C comprised three major polypeptides of 76,000, 74,000, and 30,000 daltons and a number of larger molecular mass components present in trace amounts. The NBP of ts A1S9 cells grown at the permissive temperature was similar, except that the 30,000-kilodalton polypeptide was not detected. Such enzymatically active NBPs from WT-4 and ts+ AR cells were unaffected by 100 micrograms of novobiocin per ml, whereas the analogous preparation from ts A1S9 cells was totally inhibited. On the basis of these and other considerations, it is postulated that the ts A1S9 locus of mouse L cells encodes a temperature-sensitive polypeptide that is required for normal DNA topoisomerase II activity.

Synthesis of multimeric polyoma virus DNA in mouse L-cells: role of the tsA1S9 gene product

Several different forms of progeny viral DNA can be identified in polyoma virus (Py)-infected mouse L-cells. The majority comprise mature form I superhelical DNA and the circular, double-stranded "theta" replicating intermediates in which the progeny DNA strands never exceed the unit genome length of the template. There is formed, in addition, a minority fraction of multimeric, linear, double-stranded Py DNA molecules that sediment heterogeneously at 28 to 35S and greater than 35S. Restriction enzyme analysis of these large Py DNA molecules reveals them to be tandem arrays of multiple unit genome lengths, covalently linked head to tail. It is estimated that the 28 to 35S multimeric DNA has an average size of about 20 megadaltons, made up of 6 to 20 Py genome units. The greater than 35S Py DNA is, of course, larger. Kinetic analysis indicates that formation of the monomeric progeny viral DNA and the 28 to 35S multimeric Py DNA reaches a peak at about 35 to 36 h postinfection. Synthesis of the very large linear molecules of greater than 35S is first detected after this interval and continues thereafter. The de novo synthesis of all of these progeny Py DNA molecules proceeds apparently normally in Py-infected tsA1S9 mouse L-cells incubated at 38.5 degrees C under conditions which restrict normal cellular DNA replication. These findings suggest that the cellular DNA topoisomerase II activity, encoded in the tsA1S9 locus (R. W. Colwill and R. Sheinin, submitted for publication), is not required for de novo formation of any form of Py DNA. However, the total amount made and the rate of synthesis of the large molecular weight Py DNA are affected very late in temperature-inactivated tsA1S9 cells.

Temperature-sensitive mutants of BALB/3T3 cells. III. Hybrids between ts2 and other mouse mutant cells affected in DNA synthesis and correction of ts2 defect by human X chromosome

Complementation studies were performed with ts2, a mouse 3T3 cell mutant temperature sensitive (ts) for cell and viral DNA synthesis. The ts phenotype is corrected by non-ts mouse or human cells and a non-DNA ts mutant. This gene had been localized to a region on the human X chromosome near the HPRT locus based on isozyme and karyotype analysis of hybrids. Unusually rapid loss and fragmentation of human chromosomes occurs in hybrids with ts2. Hybrids between ts2 and other DNA- ts mutants of mouse cells did not show complementation of the growth phenotype.

Inhibition of DNA chain elongation in a purine-auxotrophic mutant of Chinese hamster

DNA synthesis has been examined in a purine-auxotrophic mutant cell line of Chinese hamster (V79 pur 1) under conditions of purine deprivation. At 6 h after the removal of purines from the growth medium, there is a decrease in semiconservative DNA replication. Alkaline velocity centrifugation of the DNA synthesized during a 1-min pulse under conditions of purine deprivation shows that approximately 50% of the newly replicated DNA is the size of Okazaki pieces. These are not incorporated into bulk DNA during a 1-h chase. If the purine supply is restored to the growth medium, these short DNA pieces are jointed to full-sized DNA within 1 h. DNA fiber autoradiolgraphy reveals a retardation in the rate of DNA replication fork movement but no apparent inhibition of initiation of synthesis on replication units within clusters actively engaged in replication. Our results indicate that purine deprivation specifically inhibits elongation of nascent dna chains.

DNA synthesis in temperature-sensitive mutants of the cell cycle infected by polyoma virus and adenovirus

tsAF8 cells are a temperature-sensitive (ts) mutant of BHK cells that are arrested in G1 at the nonpermissive temperature. When made quiescent by serum restriction, they can be stimulated to enter S phase by 10% serum at 34 degrees C but not at 40.6 degrees C. The same results can be obtained if quiescent cells are infected with polyoma virus or adenovirus 12 instead of serum. However, adenovirus 2 infection stimulates DNA synthesis in tsAF8 cells at both 34 degrees C and 40.6 degrees C. The DNA synthesized after adenovirus 2 infection has been shown to be cellular DNA by CsCl density centrifugation. By density labeling it can be shown that adenovirus 2-induced DNA synthesis is due to semiconservative replication. The difference between adenovirus 2 and polyoma (or serum) is also evident with another ts mutant of BHK cells, ts13 cells. These results open the possibility of identifying the viral or cellular mechanism at the basis of this difference in the induction of host DNA synthesis between adenovirus 2 and polyoma or serum.

Mutant mouse L-cells: a model for megaloblastic anaemia

When temperature-sensitive (ts) mutant lines of mouse L-cells, ts AIS9 and ts CI, are shifted from 34C to 38.5C, a rapid inhibition of DNA synthesis and mitosis occurs. During this phase, cell and nuclear growth continues and results in a substantial increase in cell and nuclear volume. Such cellular modifications are also associated with a marked dispersal of the condensed chromatin masses of interphase nuclei, so that after 48-72 h of incubation at 38.5C, nuclear profiles of both ts cell lines bear a striking resemblance to the nuclear features characteristic of megaloblastic anaemia. Despite these marked alterations in nuclear chromatin organization, morphometric analysis indicates that the volume of condensed chromatin does not decrease. Current biochemical, cytological and morphometric data on the two ts lines of mutant mouse L-cells during expression of the mutation, suggest that they might provide a useful model to further elucidate cytological features of megaloblastic anaemia.

Structure of interphase nuclei in relation to the cell cycle. Chromatin organization in mouse L cells temperature-sensitive for DNA replication

Mutant lines of mouse L cells, TS A1S9, and TS C1, show temperature-sensitive (TS) DNA synthesis and cell division when shifted from 34 degrees to 38.5 degrees C. With TS A1S9 the decline in DNA synthesis begins after 6-8 h at 38.5 degrees C and is most marked at about 24 h. Most cells in S, G2, or M at temperature upshift complete one mitosis and accumulate in the subsequent interphase at G1 or early S as a result of expression of a primary defect, failure of elongation of newly made small DNA fragments. Heat inactivation of TS C1 cells is more rapid; they fail to complete the interphase in progress at temperature upshift and accumulate at late S or G2. Inhibition of both cell types is reversible on return to 34 degrees C. Cell and nuclear growth continues during inhibition of replication. Expression of both TS mutations leads to a marked change in gross organization of chromatin as revealed by electron microscopy. Nuclei of wild-type cells at 34 degrees and 38.5 degrees C and mutant cells at 34 degrees C show a range of aggregation of condensed chromatin from small dispersed bodies to large discrete clumps, with the majority in an intermediate state. In TS cells at 38.5 degrees C, condensed chromatin bodies in the central nuclear region become disaggregated into small clumps dispersed through the nucleus. Morphometric estimation of volume of condensed chromatin indicates that this process is not due to complete decondensation of chromatin fibrils, but rather involves dispersal of large condensed chromatin bodies into finer aggregates and loosening of fibrils within the aggregates. The dispersed condition is reversed in nuclei which resume DNA synthesis when TS cells are downshifted from 38.5 degrees to 34 degrees C. The morphological observations are consistent with the hypothesis that condensed chromatin normally undergoes an ordered cycle of transient, localized disaggregation and reaggregation associated with replication. In temperature-inactivated mutants, normal progressive disaggregation presumably occurs, but subsequent lack of chromatin replication prevents reaggregation.

Semi-conservative and non-conservative replication of DNA in temperature-sensitive mouse L-cells

The mode of DNA replication has been studied in wild-type mouse L-cells (WT-4) and in two subclones (TS A1S9 and ts C1 cells) which are temperature-sensitive in DNA synthesis. It has been demonstrated that DNA is replicated by the semi-conservative mechanism in WT-4 cells grown at 34 degrees C or at 38.5 degrees C throughout the logarithmic phase and into the stationary phase. Similar results were obtained with ts A1S9 and ts C1 cells grown at the permissive temperature (34 degrees C). When the latter cells were incubated at the non-permissive temperature (38.5 degrees C) inactivation of DNA synthesis appeared to proceed through three general stages. During the first 24 h after temperature upshift suppression of semi-conservative DNA replication occurred. During the second stage a very low level of semi-conservative synthesis was maintained. During the third stage, incorporation of dThd into DNA began to increase, often reaching 10-20% of control levels after 3-5 days. During this third stage DNA synthesis was effected by a non-conservative mechanism. Temperature-inactivated ts A1S9 cells and ts C1 cells were able to perform semi-conservative synthesis upon back-shift to 34 degrees C, using as template that DNA synthesized prior to temperature upshift.