The SbcCD protein is a member of a group of nucleases found in bacteriophag
e T4 and T5, eubacteria, archaebacteria, yeast, Drosophila, mouse and man.
Evidence from electron microscopy has revealed a distinctive structure cons
isting of two globular domains linked by a long region of coiled coil, simi
lar to that predicted for the members of the SMC family. That a nuclease sh
ould have such an unusual structure suggests that its mode of action may be
complex, Here we show that the protein degrades duplex DNA in a 3'-->5' di
rection. This degradation releases products half the length of the original
duplex suggesting simultaneous degradation from two duplex ends. This may
provide a link to the unusual structure of the protein since our data are c
onsistent with recognition and cleavage of DNA ends followed by 3'-->5' nic
king by two nucleolytic centres within a single nuclease molecule that rele
ases a half length limit product. We also show that cleavage is not simply
at the point of a single-strand/double-stand transition and that despite th
e dominant 3'-->5' polarity of degradation, a 5' single-strand can be cleav
ed when attached to duplex DNA. The implications of this mechanism for the
processing of hairpins formed during DNA replication are discussed.