Ribonucleotide Reductase reduces all four ribonucleoside diphosphates to deoxynucleoside diphosphates and functions as the major source of deoxyribo-nucleotides for DNA synthesis in vivo. Ribonucleotide reductase is cell cycle regulated and most of the increased activity during S phase results from a 6 to 10 fold increase in M2 subunit activity. Hydroxyurea resistant cell lines have increased M2 activity and M2 gene amplification but normal cell cycle regulation of ribonucleotide reductase activity. We investigated M2 specific mRNA content of cell cycle specific populations using a 1487 bp long mouse M2 cDNA to probe northern dot blots of total cellular RNA from wild type, cyclic AMP dependent protein kinase deficient, and two hydroxyurea resistant cell lines one with and one without CAMP dependent protein kinase activity. Five sequential cell cycle fractions were obtained by centrifugal elutriation. Hydroxyurea resistant cell lines had greater M2 specific RNA per mg than wild type though their cell cycle regulation appeared to be largely the same. All cell types have low concentrations of M2 specific RNA in very early Gl, a dramatic increase in late Gl/early S, a rapid decline in later S and finally a gradual rise in G2 phase. These data suggest transcriptional regulation of M2 during the cell cycle is at least in part responsible for cell cycle variation in ribonucleotide reductase activity.
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