Inhibition of DNA Synthesis and Mitosis by DNA Affinity Proteins Released from Thymus Cells during in Vitro Incubation

Abstract

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.