Identification of the Origin of Transfer ( oriT ) and DNA Relaxase Required for Conjugation of the Integrative and Conjugative Element ICE Bs1 of Bacillus subtilis

Abstract

ABSTRACT Integrative and conjugative elements (ICEs), also known as conjugative transposons, are mobile genetic elements that can transfer from one bacterial cell to another by conjugation. ICE Bs1 is integrated into the trnS-leu2 gene of Bacillus subtilis and is regulated by the SOS response and the RapI-PhrI cell-cell peptide signaling system. When B. subtilis senses DNA damage or high concentrations of potential mating partners that lack the element, ICE Bs1 excises from the chromosome and can transfer to recipients. Bacterial conjugation usually requires a DNA relaxase that nicks an origin of transfer ( oriT ) on the conjugative element and initiates the 5′-to-3′ transfer of one strand of the element into recipient cells. The ICE Bs1 ydcR ( nicK ) gene product is homologous to the pT181 family of plasmid DNA relaxases. We found that transfer of ICE Bs1 requires nicK and identified a cis -acting oriT that is also required for transfer. Expression of nicK leads to nicking of ICE Bs1 between a GC-rich inverted repeat in oriT , and NicK was the only ICE Bs1 gene product needed for nicking. NicK likely mediates conjugation of ICE Bs1 by nicking at oriT and facilitating the translocation of a single strand of ICE Bs1 DNA through a transmembrane conjugation pore.