Structural and functional insight into sugar-nonspecific nucleases in host defense.

In prokaryotes, sugar-nonspecific nucleases that cleave DNA and RNA in a sequence-independent manner take part in host defense, as well as site-specific restriction enzymes. Examples include the periplasmic nuclease Vvn and the secreted nuclease ColE7, which degrade foreign nucleic acid molecules in the host periplasm and in the cytoplasm of foreign cells, respectively. Recently determined crystal structures of Vvn and ColE7 in complex with double-stranded DNA provide structural insight into nonspecific DNA interactions and cleavage by sugar-nonspecific nucleases. Both nucleases bind DNA at the minor groove through a common 'betabetaalpha-metal' endonuclease motif and primarily contact the DNA phosphate backbone, probably to avoid sequence-dependent base recognition. In eukaryotes, several apoptotic endonucleases that are responsible for DNA degradation in programmed cell death also contain a betabetaalpha-metal fold at the active site, suggesting that they may recognize and cleave DNA in a comparable way.