The metallo-beta-lactamase fold contains five sequence motifs. The first four motifs are found in pfam00753 and are common to all metallo-beta-lactamases. This, the fifth motif, appears to be specific to Zn-dependent metallohydrolases such as ribonuclease J 2 which are involved in the processing of mRNA. This domain adds essential structural elements to the CASP-domain and is unique to RNA/DNA-processing nucleases, showing that they are pre-mRNA 3'-end-processing endonucleases.