Ribonucleoside diphosphate reductase (RR) from vaccinia virus was recently cloned and overexpressed rn Escherichia coli. The amino acid sequence identities of the small and large subunits between the mouse and the vaccinia virus reductase are approximately 80 and 72 percent, respectively. In addition, vaccinia virus RR displays similar complex allosteric regulation to the mouse enzyme and other eukaryotic reductases. The overall activity of the enzyme, which has two subunits (Rl and R2), is regulated through binding to ATP, which activates the enzyme, and dATP which seryes as an inhibitor. Both nucleotides bind to the same allosteric site, called the activity site, on the large subunit of RR. The specificity of the enzyme towards the four ribonucleoside diphosphate substrates is regulated by the binding of ATP, dATP, dTTP and GTp. Each of these nucleotides affects the reduction of a specific nucleoside diphosphate. Although this enzyme's allosteric regulation is kinetically well understood it has not been possible so far to gain further structural information about the location of the activity site and specificity site. The use of deletion mutants and photoaffinity labeling of the large subunit to identify the location of the binding sites is the incentive for this thesis. With the introduction of 6xHis/Nickel Nitrilo-tri-acetic acid (Ni-NTA) chromatography, the purification of the large subunit was improved in the E. coli and vaccinia virus/T7 RNA polymerase hybrid system. The purification of several deletion mutant forms of the large subunit was also attempted, but it was not possible to purify any of them from either of the expression systems. The purified full-length large subunit obtained with the Ni-NTA-chromatography system was used for a photoaffinity labeling experiment with [³²P]dATP and [³²P]dTTP. The labeled proteins were proteolytically digested to find out about the specificity of the labeling experiment and also to map the binding site of the nucleotide. It was found that labeling of dATP yielded few discrete bands indicating specific binding, while a comparable experiment with dTTP indicated less specific binding, based on a larger number of labeled bands. In competition experiments with non-radioactive nucleotides, vaccinia virus R1 featured the same properties as the mouse and E. coli counterparts. This is consistent with data from kinetic experiments, which also establish the same kinetic properties between vaccinia virus RR with those of mouse and E. coli (RR). To identify the sequence of the fragments carrying the label the digests were subjected to mass spectrometric analysis. However, it was not possible to determine the sequence of the labeled fragment by mass spectrometry due to poor spectral resolution. / Graduation date: 1994