Regulation of ribonucleotide reductase in response to iron deficiency.

Ribonucleotide reductase (RNR) is an essential enzyme required for DNA synthesis and repair. Although iron is necessary for class Ia RNR activity, little is known about the mechanisms that control RNR in response to iron deficiency. In this work, we demonstrate that yeast cells control RNR function during iron deficiency by redistributing the Rnr2-Rnr4 small subunit from the nucleus to the cytoplasm. Our data support a Mec1/Rad53-independent mechanism in which the iron-regulated Cth1/Cth2 mRNA-binding proteins specifically interact with the WTM1 mRNA in response to iron scarcity and promote its degradation. The resulting decrease in the nuclear-anchoring Wtm1 protein levels leads to the redistribution of the Rnr2-Rnr4 heterodimer to the cytoplasm, where it assembles as an active RNR complex and increases deoxyribonucleoside triphosphate levels. When iron is scarce, yeast selectively optimizes RNR function at the expense of other non-essential iron-dependent processes that are repressed, to allow DNA synthesis and repair.