Abstract
Syrian hamster embryo cell lines have been used as models of neoplastic progression in vitro. Changes in phenotype and biological properties have been observed in these cell lines in association with modulation of gene transcription. We explored this natural evolution of cells in culture to investigate the reported relationship between transcriptional activity and replication in early S phase. In this study we used two nontumorigenic cell lines that had either retained (supB^+^) or lost (supB^โ^) the ability to suppress tumorigenicity of malignantly transformed hamster fibroblasts (BP6T). In association with the loss of suppressor gene function, supB^โ^ cells have downregulated the expression of the H19 and tropomyosinโI (TMโI) genes, which are actively transcribed in supB^+^ cells. Synchronous populations of supB^+^ and supB^โ^ cells were pulseโlabeled with [^3^H]thymidine and bromodeoxyuridine at 1โh intervals during S phase; the replicating DNA was isolated by centrifugation in cesium chloride gradients and hybridized to ^32^Pโlabeled gene probes. No correlation was found between the timing of gene replication and the status of expression of these two genes. TMโI replicated during the first hour and H19 replicated between the second and third hours of the S phase in the expressing and nonexpressing cell lines. Immunoglobulin gene sequences, known to be lateโreplicating in fibroblasts, replicated at the end of the S phase. These results suggest that downregulation of transcription is not always accompanied by a concomitant change in time of gene replication from early to late S phase. ยฉ 1993 WileyโLiss, Inc.