Ribonucleotide reductase maintains cellular deoxyribonucleotide pools and is thus tightly regulated during the cell cycle to ensure high fidelity in DNA replication. The Sml1 protein inhibits ribonucleotide reductase activity by binding to the R1 subunit. At the completion of each turnover cycle, the active site of R1 becomes oxidized and subsequently regenerated by a cysteine pair (CX2C) at its C-terminal domain (R1-CTD). Here we show that R1-CTD acts in trans to reduce the active site of its neighboring monomer. Both Sml1 and R1-CTD interact with the N-terminal domain of R1 (R1-NTD), which involves a conserved two-residue sequence motif in the R1-NTD. Mutations at these two positions enhancing the Sml1-R1 interaction cause SML1-dependent lethality. These results point to a model whereby Sml1 competes with R1-CTD for association with R1-NTD to hinder the accessibility of the CX2C motif to the active site for R1 regeneration.