a toxin–antitoxin system promotes the maintenance of an integrative conjugative elementtoxin-antitoxin系统促进维护一个综合共轭元素.pdfVIP

A Toxin–Antitoxin System Promotes the Maintenance of an Integrative Conjugative Element Rachel A. F. Wozniak1,2,3, Matthew K. Waldor1,2,3* 1 Channing Laboratory, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, United States of America, 2 Howard Hughes Medical Institute, Chevy Chase, Maryland, United States of America, 3 Program in Genetics, Tufts University School of Medicine, Boston, Massachusetts, United States of America Abstract SXT is an integrative and conjugative element (ICE) that confers resistance to multiple antibiotics upon many clinical isolates of Vibrio cholerae. In most cells, this ,100 Kb element is integrated into the host genome in a site-specific fashion; however, SXT can excise to form an extrachromosomal circle that is thought to be the substrate for conjugative transfer. Daughter cells lacking SXT can theoretically arise if cell division occurs prior to the element’s reintegration. Even though ,2% of SXT- bearing cells contain the excised form of the ICE, cells that have lost the element have not been detected. Here, using a positive selection-based system, SXT loss was detected rarely at a frequency of ,161027. As expected, excision appears necessary for loss, and factors influencing the frequency of excision altered the frequency of SXT loss. We screened the entire 100 kb SXT genome and identified two genes within SXT, now designated mosA and mosT (for maintenance of SXT Antitoxin and Toxin), that promote SXT stability. These two genes, which lack similarity to any previously characterized genes, encode a novel toxin-antitoxin pair; expression of mosT greatly impaired cell growth and mosA expression ameliorated MosT toxicity. Factors that promote SXT excision upregulate mosAT expression. Thus, when the element is extrachromosomal and vulnerable to l