This domain can be found in 5' to 3' exoribonuclease 1 (XRN1) which belong to a family of conserved enzymes in eukaryotes and have important functions in transcription, RNA metabolism, and RNA interference. Xrn1 in fungi and animals is primarily cytosolic and is involved in degradation of decapped mRNAs, nonsense mediated decay, microRNA decay and is essential for proper development. The Xrn1 homolog in Drosophila, known as Pacman, is required for male fertility [1]. This domain (D1) along with 3 other domains, make up a 510-residue segment following the conserved regions found in XRNs but they are only present in XRN1 and are absent in Rat1/XRN2. The amino acid sequences of these four domains contain an excess of basic residues, suggesting that these domains might help in binding the RNA substrate. Mutational studies carried out in D1 domain show that the mutant forms had dramatically reduced nuclease activity towards ssDNA substrate indicating that domain D1 is required for Xrn1 nuclease activity [1].

1: Structural and biochemical studies of the 5'-->3' exoribonuclease Xrn1. Chang JH, Xiang S, Xiang K, Manley JL, Tong L; Nat Struct Mol Biol. 2011;18:270-276. PMID:21297639

2: Coupled 5' nucleotide recognition and processivity in Xrn1-mediated mRNA decay. Jinek M, Coyle SM, Doudna JA; Mol Cell. 2011;41:600-608. PMID:21362555