Replication of proto-oncogenes early during the S phase in mammalian cell lines

Members of several classes of proto-oncogenes replicate during the first third of S-phase in two human (K562 erythroleukemia and HeLa), one Chinese hamster (CHO) and eight mouse cell lines. These cell lines exhibit a variety of specialized functions characteristic of pre-B and B cells, T cells and erythroid cells. The proto-oncogenes studied include fos, myc, myb, abl, fes, fms, mos, raf, rel, sis, Ha-ras, Ki-ras, and N-ras. In K562 cells, amplified and rearranged c-abl genes show a pattern of temporal replication during S that is similar to the pattern observed for the 5' breakpoint cluster region (bcr) and the amplified C lambda light chain immunoglobulin genes. The c-Ki-ras related sequences in CHO cells provide one example of late replicating proto-oncogene sequences that are present in multiple copies. The cellular gene N-myc replicates late during S in some of these cell lines. In three pre-B cell lines in which N-myc specific transcripts have been detected, N-myc replicates earlier in the S phase than in the other cell lines studied here.