UNEXPECTED SEQUENCE SIMILARITY BETWEEN NUCLEOSIDASES AND PHOSPHORIBOSYLTRANSFERASES OF DIFFERENT SPECIFICITY

Amino acid sequences of enzymes that catalyze hydrolysis or phosphorol ysis of the N-glycosidic bond in nucleosides and nucleotides (nucleosi dases and phosphoribosyltransferases) were explored using computer met hods for database similarity search and multiple alignment. Two new fa milies, each including bacterial and eukaryotic enzymes, were identifi ed. Family I consists of Escherichia coli AMP hydrolase (Amn), uridine phosphorylase (Udp), purine phosphorylase (DeoD), uncharacterized pro teins from E. coli and Bacteroides uniformis, and, unexpectedly, a gro up of plant stress-inducible proteins. It is hypothesized that these p lant proteins have evolved from nucleosidases and may possess nucleosi dase activity. The proteins in this new family contain 3 conserved mot ifs, one of which was found also in eukaryotic purine nucleosidases, w here it corresponds to the nucleoside-binding site. Family II is compr ised of bacterial and eukaryotic thymidine phosphorylases and anthrani late phosphoribosyltransferases, the relationship between which has no t been suspected previously. Based on the known tertiary structure of E. coli thymidine phosphorylase, structural interpretation was given t o the sequence conservation in this family. The highest conservation i s observed in the N-terminal alpha-helical domain, whose exact functio n is not known. Parts of the conserved active site of thymidine phosph orylases and anthranilate phosphoribosyltransferases were delineated. A motif in the putative phosphate-binding site is conserved in family II and in other phosphoribosyltransferases. Our analysis suggests that certain enzymes of very similar specificity, e.g., uridine and thymid ine phosphorylases, could have evolved independently. In contrast, enz ymes catalyzing such different reactions as AMP hydrolysis and uridine phosphorolysis or thymidine phosphorolysis and phosphoribosyl anthran ilate synthesis are likely to have evolved from common ancestors.