Lee Catherine A, Grossman Alan D
Department of Biology, Building 68-530, MIT, Cambridge, MA 02139, USA.
J Bacteriol. 2007 Oct;189(20):7254-61. doi: 10.1128/JB.00932-07. Epub 2007 Aug 10.
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. ICEBs1 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, ICEBs1 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 ICEBs1 ydcR (nicK) gene product is homologous to the pT181 family of plasmid DNA relaxases. We found that transfer of ICEBs1 requires nicK and identified a cis-acting oriT that is also required for transfer. Expression of nicK leads to nicking of ICEBs1 between a GC-rich inverted repeat in oriT, and NicK was the only ICEBs1 gene product needed for nicking. NicK likely mediates conjugation of ICEBs1 by nicking at oriT and facilitating the translocation of a single strand of ICEBs1 DNA through a transmembrane conjugation pore.
整合与接合元件（ICEs），也被称为接合转座子，是一类可通过接合作用从一个细菌细胞转移至另一个细菌细胞的移动遗传元件。ICEBs1整合于枯草芽孢杆菌的trnS-leu2基因中，并受SOS应答以及RapI-PhrI细胞间肽信号系统调控。当枯草芽孢杆菌感知到DNA损伤或存在高浓度缺乏该元件的潜在交配伙伴时，ICEBs1会从染色体上切除并转移至受体细胞。细菌接合通常需要一种DNA松弛酶，它在接合元件上切割转移起始位点（oriT），并启动该元件一条链从5'到3'向受体细胞的转移。ICEBs1的ydcR（nicK）基因产物与质粒DNA松弛酶的pT181家族同源。我们发现ICEBs1的转移需要nicK，并鉴定出一个转移所需的顺式作用oriT。nicK的表达导致ICEBs1在oriT中富含GC的反向重复序列之间发生切割，且NicK是切割所需的唯一ICEBs1基因产物。NicK可能通过在oriT处切割并促进ICEBs1单链DNA通过跨膜接合孔的易位来介导ICEBs1的接合。
