Effect of the myb gene product on expression of the PCNA gene in fibroblasts.

Tk-ts13 cells are BHK-derived fibroblasts that have a G1-specific temperature sensitive (ts) mutation so that the cells arrest in the G1 phase of the cell cycle at the restrictive temperature of 39.6 degrees. We have introduced into these cells a plasmid carrying the human c-myb cDNA under the control of the early SV40 promoter. The resulting cell line, ts13 myb cells, express the c-myb RNA and protein and, when serum-stimulated, they undergo one round of DNA replication even at the restrictive temperature. Under the same conditions, the parent cell line (tk-ts13 cells) are blocked in G1 and fail to replicate DNA. We have investigated the expression of two late growth-regulated genes, PCNA and histone H3, in tk-ts13 and in ts 13 myb cells, both at permissive and restrictive temperature. At the permissive temperature of 34 degrees, the mRNA levels for PCNA and histone H3 increase markedly after serum stimulation in both cell lines, reaching a peak at the time of DNA synthesis. At the restrictive temperature of 39.6 degrees, the mRNA's for PCNA, and histone H3 are detectable in serum-stimulated ts13 myb cells while they are not detectable in the parent tk-ts13 cell line. Run-on transcription assays indicate that these 2 genes are transcribed in tk-ts13 cells equally well at permissive and nonpermissive temperatures, and that the presence of the myb product does not significantly increase their rates of transcription. The stability of the respective mRNA's is roughly the same at either temperature and in both types of cells. These results indicate: (1) a constitutively expressed c-myb gene product confers to fibroblasts the ability of temporarily by-passing a ts block in the G1 phase of the cell cycle and (2) the myb product, under these conditions, regulates the mRNA levels of PCNA and histone H3 either directly or indirectly by a post-transcriptional mechanism.