A 5-NUCLEOTIDE SEQUENCE PROTECTS DNA FROM EXONUCLEOLYTIC DEGRADATION BY ADDAB, THE RECBCD ANALOG OF BACILLUS-SUBTILIS

Homologous recombination in Bacillus subtilis requires the product of the addA and addB genes, the AddAB enzyme, This enzyme, which is both a helicase and a powerful nuclease, is thought to be the counterpart o f the Escherichia coli RecBCD enzyme, From this analogy, it is expecte d that the nuclease activity of AddAB can be downregulated by a specif ic DNA sequence, which would correspond to the chi site in E. coli, Us ing protection of linear double-stranded DNA as a criterion, we identi fied the five-nucleotide sequence 5'-AGCGG-3', or its complement B'-CC GCT-3', as being sufficient for AddAB nuclease attenuation, We have sh own further that this attenuation occurs only if the sequence is prope rly oriented with respect to the translocating AddAB enzyme, Finally, inspection of the complete B. subtilis genome revealed that this five- nucleotide sequence is over-represented and is, in a majority of cases , co-oriented with DNA replication. Based on these observations, we pr opose that 5'-AGCGG-3', or its complement, is the B. subtilis analogue of the E. coli chi sequence.