E. coli ribonucleotide reductase (RNR), composed of the homodimeric subunits alpha2 and beta2, catalyzes the conversion of nucleotides to deoxynucleotides via complex radical chemistry. The radical initiation process involves a putative proton-coupled electron transfer (PCET) pathway over 35 A between alpha2 and beta2. Y356 in beta2 has been proposed to lie on this pathway. To test this model, intein technology has been used to make beta2 semi-synthetically in which Y356 is replaced with a DOPA-amino acid. Analysis of this mutant with alpha2 and various combinations of substrate and effector by SF UV-vis spectroscopy and EPR methods demonstrates formation of a DOPA radical concomitant with disappearance of the tyrosyl radical, which initiates the reaction. The results reveal that Y356 lies on the PCET pathway and demonstrate the first kinetically competent conformational changes prior to ET. They further show that substrate binding brings about rapid conformational changes which place the complex into its active form(s) and suggest that the RNR complex is asymmetric.