Mlu1 cell cycle box (MCB) elements are found near the start site of yeast genes expressed at G1/S. Basal promoters dependent on the elements for upstream activating sequence activity are inactive in Deltaswi6 yeast. Yeast were screened for mutations that activated MCB reporter genes in the absence of Swi6. The mutations identified a single complementation group. Functional cloning revealed the mutations were alleles of the TRR1 gene encoding thioredoxin reductase. Although deletion of TRR1 activated MCB reporter genes, high copy expression did not suppress reporter gene activity. The trr1 mutations strongly (20-fold) stimulated MCB- and SCB (Swi4/Swi6 cell cycle box)-containing reporter genes, but also weakly (3-fold) stimulated reporter genes that lacked these elements. The trr1 mutations did not affect the level or periodicity of three endogenous MCB gene mRNAs (TMP1, RNR1, and SWI4). Deletion of thioredoxin genes TRX1 and TRX2 recapitulated the stimulatory effect of trr1 mutations on MCB reporter gene activity. Conditions expected to oxidize thioredoxin (exposure to H2O2) induced MCB gene expression, whereas conditions expected to conserve thioredoxin (exposure to hydroxyurea) inhibited MCB gene expression. The results suggest that thioredoxin oxidation contributes to MCB element activation and suggest a link between thioredoxin-oxidizing processes such as ribonucleotide reduction and cell cycle-specific gene transcription.