THE SBCCD NUCLEASE OF ESCHERICHIA-COLI IS A STRUCTURAL MAINTENANCE OFCHROMOSOMES (SMC) FAMILY PROTEIN THAT CLEAVES HAIRPIN DNA

Hairpin structures can inhibit DNA replication and are intermediates i n certain recombination reactions. We have shown that the purified Sbc CD protein of Escherichia coli cleaves a DNA hairpin. This cleavage do es not require the presence of a free (3' or 5') DNA end and generates products with 3'-hydroxyl and 5'-phosphate termini. Electron microsco py of SbcCD has revealed the ''head-rod-tail'' structure predicted for the SMC (structural maintenance of chromosomes) family of proteins, o f which SbcC is a member. This work provides evidence consistent with the proposal that SbcCD cleaves hairpin structures that halt the progr ess of the replication fork, allowing homologous recombination to rest ore DNA replication.