Phenotypes of c-Myc-deficient rat fibroblasts isolated by targeted homologous recombination.

Rat fibroblast cell lines with targeted disruptions of both c-myc gene copies were constructed. Although c-myc null cells are viable, their growth is significantly impaired. The absence of detectable N-myc or L-myc expression indicates that Myc function is not absolutely essential for cell viability. The c-myc null phenotype is stable and can be reverted by introduction of a c-myc transgene. Exponentially growing c-myc null cells have the same cell size, rRNA, and total protein content as their c-myc +/+ parents, but the rates of RNA and protein accumulation as well as protein degradation are reduced. Both the G1 and G2 phases of the cell cycle are significantly lengthened, whereas the duration of S phase is unaffected. This is the first direct demonstration of a requirement for c-myc in G2. The G0-->S transition is synchronous, but S-phase entry is significantly delayed. The c-myc null cell lines reported here are a new experimental system in which to investigate the importance of putative c-Myc target genes and to identify novel downstream genes involved in cell cycle progression and apoptosis.