Fibroblast-like cells from a BALB/c mouse embryo were propagated and transferred weekly in culture with a constant inoculation density of 5 × 10³ cells/cm². In this culture schedule a period of decreased proliferation (crisis) continued for 3 to 4 weeks before the initiation of continuous cell proliferation. This provided an experimental system suitable for studying proliferation conversion. The number and the proliferation rate of clonogenic cells increased early in crisis. Production of the colony-stimulating factor for mouse bone marrow cells by fibroblast-like cells was enhanced during the period of decreasing proliferation and crisis, then it decreased when continuous proliferation began. The results indicate that cells with infinite proliferation potential appear early in crisis, although the major population of cells terminates proliferation during crisis and enhances differentiated functions.

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During in vitro incubation of thymus cells, proteins with anaffinity for single stranded DNA (ss-DNA) are released into the medium. In competitive experiments with unlabeled DNA, the ss-DNA-binding proteins showed a high specificity toward ss-DNA. However, they did not recognize the difference between heterologous and homologous ss-DNA. Thymus cells were incubated in MEM for 3 h at 37°C. The ss-DNA-binding proteins in the cell culture medium then were purified by ss-DNA-cellulose chromatography. The effect of the ss-DNA-binding proteins on lymphocytes or nonlymphocytes was studied in vitro by the incorporation of [3H]-thymidine. The addition of ss-DNA binding proteins to the assay system markedly inhibited the DNA, synthesis of nonactivated spleen cells. Spleen cells activated with phytohemagglutinin also were inhibited by the addition of ss-DNA-binding proteins. Further experiments showed that the DNA-binding proteins inhibited the DNA-synthesis of L-cells ; this activity was heat labile. The proteins also inhi-bited the mitosis of L-cells.

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Cytoskeletal networks composed of microtubules (MTs), neurofilaments (NFs), and cross-bridges were well preserved in giant axons ofthe squid by adding dimethylsulfoxide (DMSO) to the glutaraldehyde solutionfor fixation. DMSO remarkably supported preservation of the otherwise labile MTs in the giant axon. Bundles of MTs, frequently associated with NFs, wereobserved in the axoplasm running parallel to the longitudinal axis of the axon. The distribution of MTs in the axon was more prominent in the peripheral axoplasm (60-80 MTs/μm2) than in the interior (20-25 MTs/μm2). At the edge of the periphery of the axoplasm, some MTs seemed to end in the axolemma, but others appeared to be cross-bridged to the axolemma by filamentous structures. In axons that had been treated with cold or had been injured, winding tubules thinner than the MT appeared in the peripheral axoplasm.

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Eighty four precent of the total extractable DNA polymerasea α activity was associated with nuclei isolated from mouse myeloma MOPC 104E cells in the presence of Ca²⁺. From the nuclei, DNA polymerase α was extracted from these nuclei in the three separate steps : activity I, KCl free medium (minus Ca²⁺); II, 100 mM KCl; III, 200 mM KCl. Partially purified DNA polymerase a was eluted from a native or denatured DNA-cellulose colu-mn by 100 mM KCl. This conforms very well to the KCl concentration requiredto extract activity II from nuclei. Therefore, activity II is thought to be bound to the DNA in the nucleus. By contrast the activity I may be in a free form.After extraction, activity III was eluted from DNA-cellulose by the same KCl concentration as were I or II. This indicates that activity III did not have astronger binding ability than I or II to DNA. Activity III may be present in nucleus by binding to specific sites of DNA or its making a complex with somefactor(s). The KCl concentrations required to extract DNA polymerase β and γ conformed to those required to elute these enzymes from a DNA-cellulosecolumn. This evidence suggests that most DNA polymerase β and nuclear DNA polymerase γ made a complex with the DNA in nuclei.
DNA polymerase α was resolved into three active forms, A, B and C in the order of their elution from DEAE-cellulose as described previously (13, 21).The activity I described above was composed mainly of form C while the majoractivity of II and III was form B. The DNA polymerase α present in the cytoplasm soluble fraction contained forms A and C. Thus, form B of apolymerase forms a complex with some nuclear structure, presumably DNA, and probably is an active form involved in DNA synthesis.

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Actin was isolated from myxoamoebae (the haploid vegetativephase) of Physarum polycephalum by a simple method, employing gel filtration on Sephadex G-100. This procedure will be useful for isolating both actin and myosin from the same myosin B fraction.
The purified myxoamoeba actin was homogeneous electrophoretically and contained no β-actinin-like protein. In addition, myxoamoeba actin showedproperties similar to other actins; the polymerization of G-actin with 0.1 MKC1, the reduced viscosity of F-actin, the activation of myosin ATPase by actin, and the forming of long helical filaments with 0.1 M KC1 and arro-whead-like structures with heavy meromyosin.
The amino acid composition of myxoamoeba actin resembled that of theplasmodium (the diploid vegetative phase) of Physarum, except that its glycine content was higher.

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We isolated several mutants that are highly resistant to 2'deoxy-2'-azidocytidine by culturing mutagenized mouse FM3A cells in a medium containing 10^-4 M of this analogue. The mutants incorporated deoxycytidine into acid-insoluble materials at a reduced rate, but incorporated cytidine normally. All the mutants isolated also were resistant to 1-β-D-arabinofuranosylcytosine. The activity of deoxycytidine kinase in the crude extracts of these mutants was reduced to about 10 % that of the FM3A cells. Ourresults indicate that 2'-deoxy-2'-azidocytidine is recognized as deoxyribonucleoside by deoxycytidine kinase in contrast to reports that phosphate forms of this nucleoside analogue are recognized as ribonucleotide by the ribonucleotide reductase from calf thymus and E. coli and by E. coli primase.

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The immunofluorescent method using antiserum against the nonhistone protein-DNA complex of Syrian hamster cells has shown that DNA-bound nonhistone chromosomal proteins in baby hamster kidney cells are released into the cytoplasm during mitosis but have returned to the nucleus and are incorporated in daughter cell nuclei when cell division has finished.

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The arrangement of myofilaments in the smooth muscle of guinea pig taenia coli was observed by electron microscopy to be markedly changed more than 30 min after the oneset of K⁺-induced isometric contraction. During the tonic response the isometric tension gradually increased, nevertheless almost all the thick filaments disappeared from the whole cell, and thin filaments were located only around the submembraneous area. Using the chemically skinned fiber, we confirmed that the thick filaments had disintegrated and that there was a slight decrease in intracellular Mg²⁺ concentration.

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