A gene near the plasmid pSa origin of replication encodes a nuclease

We have cloned and sequenced a gene (nuc) from the IncW plasmid pSa which shows amino acid sequence similarity to staphylococcal nuclease (EC 3.1.4.7) and to the parB locus of plasmid RP4. The 525 bp open reading frame encodes a 174-amino-acid potential polypeptide of 19.7 kDa. Expression of the gene was confirmed using an in vitro transcription-translation assay which produced a protein of identical size. Nuclease activity was demonstrated using DNA as the substrate in toluidine blue-DNA agar plates. The deduced amino acid sequence revealed a signal sequence, and TnphoA insertion within the open reading frame indicated that a portion of the protein is transported across the bacterial cell membrane.