title: Functional characterization of an alkaline exonuclease and single strand annealing protein from the SXT genetic element of Vibrio cholerae.
abstract: BACKGROUND
  SXT is an integrating conjugative element (ICE) originally isolated from Vibrio cholerae, the bacterial pathogen that causes cholera. It houses multiple antibiotic and heavy metal resistance genes on its ca. 100 kb circular double stranded DNA (dsDNA) genome, and functions as an effective vehicle for the horizontal transfer of resistance genes within susceptible bacterial populations. Here, we characterize the activities of an alkaline exonuclease (S066, SXT-Exo) and single strand annealing protein (S065, SXT-Bet) encoded on the SXT genetic element, which share significant sequence homology with Exo and Bet from bacteriophage lambda, respectively.
  
  
  RESULTS
  SXT-Exo has the ability to degrade both linear dsDNA and single stranded DNA (ssDNA) molecules, but has no detectable endonuclease or nicking activities. Adopting a stable trimeric arrangement in solution, the exonuclease activities of SXT-Exo are optimal at pH 8.2 and essentially require Mn2+ or Mg2+ ions. Similar to lambda-Exo, SXT-Exo hydrolyzes dsDNA with 5'- to 3'-polarity in a highly processive manner, and digests DNA substrates with 5'-phosphorylated termini significantly more effectively than those lacking 5'-phosphate groups. Notably, the dsDNA exonuclease activities of both SXT-Exo and lambda-Exo are stimulated by the addition of lambda-Bet, SXT-Bet or a single strand DNA binding protein encoded on the SXT genetic element (S064, SXT-Ssb). When co-expressed in E. coli cells, SXT-Bet and SXT-Exo mediate homologous recombination between a PCR-generated dsDNA fragment and the chromosome, analogous to RecET and lambda-Bet/Exo.
  
  
  CONCLUSIONS
  The activities of the SXT-Exo protein are consistent with it having the ability to resect the ends of linearized dsDNA molecules, forming partially ssDNA substrates for the partnering SXT-Bet single strand annealing protein. As such, SXT-Exo and SXT-Bet may function together to repair or process SXT genetic elements within infected V. cholerae cells, through facilitating homologous DNA recombination events. The results presented here significantly extend our general understanding of the properties and activities of alkaline exonuclease and single strand annealing proteins of viral/bacteriophage origin, and will assist the rational development of bacterial recombineering systems.
authors.affiliation: Department of Biochemistry, The Chinese University of Hong Kong, Shatin, Hong Kong.
authors.forename: Wen-yang
authors.initials: WY
authors.lastname: Chen
authors.forename: John Ws
authors.initials: JW
authors.lastname: Ho
authors.forename: Jian-dong
authors.initials: JD
authors.lastname: Huang
authors.forename: Rory M
authors.initials: RM
authors.lastname: Watt
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D001483:Base Sequence; D004269:DNA, Bacterial / Q000235:genetics / Q000378:metabolism; D003851:Deoxyribonucleases / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000378:metabolism / Q000494:pharmacology; D005090:Exodeoxyribonucleases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D023061:Gene Order; D007287:Inorganic Chemicals / Q000378:metabolism / Q000494:pharmacology; D008969:Molecular Sequence Data; D010710:Phosphates / Q000378:metabolism / Q000494:pharmacology; D010957:Plasmids / Q000235:genetics; D055503:Protein Multimerization; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D011995:Recombination, Genetic; D016415:Sequence Alignment; D014734:Vibrio cholerae / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism*; D014764:Viral Proteins / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; D004791:Enzyme Inhibitors; D007287:Inorganic Chemicals; D010710:Phosphates; D011994:Recombinant Proteins; D014764:Viral Proteins; D003851:Deoxyribonucleases; D005090:Exodeoxyribonucleases; C043586:exo protein, Bacteriophage lambda