ABSTRACT
Cells with increased ribonucleotide reductase activity are unique among drug-resistant cell lines with amplified copies of specific genes because ribonucleotide reductase activity is dependent on two different gene products. These are the two nonidentical subunits of ribonucleotide reductase, proteins R1 and R2. During the stepwise selection of cells resistant to increasing concentrations of hydroxyurea, the degree of R2 gene amplification corresponded to the degree of ornithine decarboxylase gene amplification, indicating coamplification of the two genes. The active R2 gene was assigned to chromosome 12 while the other R2-related sequences represent pseudogenes. The levels of R1/R2 mRNA in resting cells are very low, and therefore a very sensitive antisense RNA, solution hybridization, RNase protection assay was developed, capable of detecting down to 1 molecule of R1/R2 mRNA per cell. The R1 and R2 subunits are present in nonstoichiometric amounts in most cells, with the R2 subunit always being limiting.
