A membrane-altered mutant cold-sensitive for growth

Cell division cycle in mammalian cells. VIII. Mapping of G1 into six segments using temperature-sensitive cell cycle mutants

The G1 blocks in three temperature-sensitive (ts) Syrian hamster cell-cycle mutants have been mapped in relation to other G1 landmarks. Two mutants reported here, ts-559 and ts-694, show defective progression only in G1. When shifted from the permissive temperature of 33 degrees C to the non-permissive temperature of 39 degrees C, G1 cells of these two mutants show no further cell cycle progression, while cells in S, G2 and mitosis progress through the cell cycle but become blocked after entering G1. The two mutants complement each other, and also complement the previously reported mutant ts-550C with blocks in both G1 and G2 of the cell cycle. The locations of the G1 blocks in both ts-559 and ts-694 are before the hydroxyurea arrest point. The G1 ts point in ts-694 is prior to the isoleucine deprivation and serum starvation points, while the G1 block in ts-559 is after the serum starvation point but before the isoleucine block. Other G1 block points which have been reported are in mutants of different species and isolated in different laboratories, causing difficulties for relative positioning of the blocks in G1. The mutants for mapping in this study have been isolated from the same cell line. The G1 ts arrest points of ts-559 and ts-694, and that found in ts-550C, together with nutritional deprivations and metabolic inhibitors, provide seven reference points which divide G1 into six segments, each of which is bracketed by two adjacent points: mitosis, ts-694 block, serum starvation arrest point, ts-559 block, isoleucine deprivation arrest point, ts-550C block, hydroxyurea or excess-thymidine arrest segment.

Studies on cell division in mammalian cells: VI. A temperature-sensitive mutant blocked in both G1 and G2 phases of the cell cycle

A temperature-sensitive mammalian cell cycle mutant with blocks in G1 and G2 phases of the cell cycle has been isolated in culture. When shifted from the permissive temperature of 33 degrees C to the nonpermissive temperature of 39 degrees C, the fraction of cells initiating DNA synthesis as well as the fraction of cells entering mitosis decreased rapidly. Combined cytophotometric and autoradiographic analysis on the cells at 39 degrees C showed that G1 cells, with the exception of those in late G1, were arrested in that phase. Cells is S phase at the time of temperature shift, together with the late g1 cells which subsequently entered S, continued through S into G2, but were blocked in that phase of the cell cycle and unable to initiate mitosis. Those cells already in mitosis completed cell division at 39 degrees C. The G1 block point of ts-550C was found to be located after the serum starvation and isoleucine deprivation arrest points, approximately 3 h before initiation of DNA synthesis.

Dominant versus recessive behavior of a cold- and a heat-sensitive mammalian cell cycle variant in heterokaryons

A heat-sensitive (hs, arrested at 39.5 degrees C, termed 21-Ta) and a cold-sensitive (cs, arrested at 33 degrees C, termed 21-Fb) clonal cell cycle variant were isolated from the same clone of the P-815 murine mastocytoma line. At the respective nonpermissive temperatures, both the hs and the cs variant were reversibly arrested in G1 phase, and numbers of cells forming colonies upon reincubation at the permissive temperature remained nearly constant for at least 6 days. Cells arrested in G1 by incubation at the respective nonpermissive temperatures were fused to cells of another P-815 clone (31-S) that had been arrested by serum deprivation. Upon reincubation in medium containing 10% serum for 48 h at 39.5 degrees C, 21-Ta x 31-S heterokaryons, similar to 31-S x 31-S homokaryons, entered the S phase, whereas at 33 degrees C, 21-Fb x 31-S heterokaryons, similar to 21-Fb x 21-Fb homokaryons, remained arrested in G1, indicating a recessive expression of the hs and a dominant expression of the cs phenotype.

Rapid, quantitative analysis of cell cycle stages of cold-sensitive derivatives of the Chinese hamster cell line CHO-K1

Cell cycle parameters of two cold-sensitive Chinese hamster cell lines have been determined by flow cytometric analysis of cell populations stained with the DNA specific fluorochrome mithramycin. The most striking finding is a depletion of S phase cells, detectable as early as 12 h after a shift to the nonpermissive temperature of 30.0 degrees C and complete by 24 h following the shift. There is a substantial increase in the proportion of cells in the G1 phase of the cell cycle compared to wild type cells under identical conditions but, surprisingly, the proportion of cells having a G2/M DNA content is quite similar in the two populations. The proportion of tetraploid cells present in these populations is not sufficient to account for this observation. Reversibility of the cold-induced block was tested by returning cells held for three days at the nonpermissive temperature to the permissive temperature. Cells having a G1 content of DNA do reenter the S phase, beginning approximately 8 h after a return to the permissive temperature.