Ribonucleotide reduction in Pseudomonas species: Simultaneous presence of active enzymes from different classes

Three separate classes of ribonucleotide reductases exist in nature. They d iffer widely in protein structure, Class I enzymes are found in aerobic bac teria and eukaryotes; class II enzymes are found in aerobic and anaerobic b acteria; class III enzymes are found in strict and facultative anaerobic ba cteria. Usually, but not always, one organism contains only one or two (in facultative anaerobes) classes. Surprisingly, the genomic sequence of Pseud omonas aeruginosa contains sequences for each of the three classes. Here, w e show by DNA hybridization that other species of Pseudomonas also contain the genes for three classes. Extracts from P. aeruginosa and P. stutzeri gr own aerobically or microaerobically contain active class I and II enzymes, whereas we could not demonstrate class III activity, Unexpectedly, class I activity increased greatly during microaerobic conditions. The enzymes were separated, and the large proteins of the class I enzymes were obtained in close to homogeneous form, The catalytic properties of all enzymes are simi lar to those of other bacterial reductases, However, the Pseudomonas class I reductases required the continuous presence of oxygen during catalysis, u nlike the corresponding Escherichia coil enzyme hut similar to the mouse en zyme, In similarity searches, the amino acid sequence of the class I enzyme of P. aeruginosa was more related to that of eukaryotes than to that of E. coli or other proteobacteria, with the large protein showing 42% identity to that of the mouse, suggesting the possibility of a horizontal transfer o f the gene. The results raise many questions concerning the physiological f unction and evolution of the three classes in Pseudomonas species.