The T4 bacteriophage gene e.1 was cloned into an expression vector and expressed in Escherichia coli, and the purified protein was identified as a Nudix hydrolase active on FAD, adenosine 5'-triphospho-5'-adenosine (Ap(3)A), and ADP-ribose. Typical of members of the Nudix hydrolases, the enzyme has an alkaline pH optimum (pH 8) and requires a divalent cation for activity that can be satisfied by Mg(2+) or Mn(2+). For all substrates, AMP is one of the products, and unlike most of the other enzymes active on Ap(3)A, the T4 enzyme hydrolyzes higher homologues including Ap(4-6)A. This is the first member of the Nudix hydrolase gene superfamily identified in bacterial viruses and the only one present in T4. Although the protein was predicted to be orthologous to E. coli MutT on the basis of a sequence homology search, the properties of the gene and of the purified protein do not support this notion because of the following. (a) The purified enzyme hydrolyzes substrates not acted upon by MutT, and it does not hydrolyze canonical MutT substrates. (b) The e.1 gene does not complement mutT1 in vivo. (c) The deletion of e.1 does not increase the spontaneous mutation frequency of T4 phage. The properties of the enzyme most closely resemble those of Orf186 of E. coli, the product of the nudE gene, and we therefore propose the mnemonic nudE.1 for the T4 phage orthologue.