Cloning of a 4.3 kb DNA fragment from Corynebacterium (formerly Brevibacterium) ammoniagenes ATCC 6872 allowed sequencing and homologous expression of the nrdF gene encoding the metallo-cofactor (CA2 protein) of the Mn-RRase. Due to a multi copy effect transconjugants of C. ammoniagenes showed a sixfold increased resistance towards the radical-scavenger hydroxyurea (HU). The organic radical in the purified CA2 protein generated a visible absorption at 437 rim, absent upon manganese-depletion, and a distinct fluorescence at 513 nm. The two signals were each quenched by HU.