BACKGROUND BACTERIA EXCHANGE GENES BY THREE CANONICAL MECHANISMS: NATURAL TRANSFORMATION CONJUGATION AND TRANSDUCTION. ARCHAEA UNDERGO TREMENDOUS AMOUNTS OF HORIZONTAL GENE TRANSFER (HGT) AND HALOARCHAEA SPECIFICALLY ARE WELL DOCUMENTED TO HAVE I) EXCHANGED GENES WITHIN SPECIES AT RATES THAT RANDOMIZE ALLELES WITHIN POPULATIONS (LIKE SEXUALLY REPRODUCING EUKARYOTES); II) EXCHANGE GENES AT RATES HIGHER THAN THOSE SEEN BETWEEN BACTERIAL MODEL SPECIES AND GENERA; III) HAVE ACQUIRED THOUSANDS OF GENES FROM BACTERIAL SOURCES E.G. THEIR AEROBIC RESPIRATION COMES LARGELY FROM BACTERIA. HOWEVER THE MECHANISMS FOR HOW THEY DO IT AND THE EXTENT TO WHICH THEY DO IT ARE LARGELY UNCHARACTERIZED. USING THE MODEL ARCHAEON HALOFERAX VOLCANII WE HAVE PREVIOUSLY SHOWN A CELL-CONTACT DEPENDENT MECHANISM (CALLED MATING) FOR GENE EXCHANGE MOVES>500KB FRAGMENTS BETWEEN SPECIES AT A RATE HIGHER THAN SEEN IN BACTERIAL MODEL ORGANISMS. RECENTLY WE HAVE BEGUN TO UNRAVEL THE MECHANISMS BEHIND GENE TRANSFER IN H. VOLCANII. OUR GOALS IN THIS PROPOSAL ARE TO IDENTIFY AND CHARACTERIZE GENES RESPONSIBLE FOR HGT IN THE ARCHAEON H. VOLCANII; TO IDENTIFY AND CHARACTERIZE COMPONENTS OF QUORUM SENSING AND IF IT REGULATES MATING; DETERMINE RATES OF HGT WITHIN AND BETWEEN HALOARCHAEAL SPECIES; AND TO ASSESS THE ROLES OF HOSTIMMUNE SYSTEMS ON GENE TRANSFER. THE SPECIFIC OBJECTIVES OF THE RESEARCH ARE: 1) IDENTIFY AND CHARACTERIZE MOLECULAR COMPONENTS NECESSARY FOR THE CELL-CELL CONTACT MEDIATED GENE EXCHANGE (MATING) IN H. VOLCANII. 2) IDENTIFY AND CHARACTERIZE COMPONENTS OF A HOMOSERINE LACTONE BASED QUORUM SENSING SYSTEM IN H. VOLCANII 3) EXAMINE FREQUENCIES OF H. VOLCANII FOR INTRA- VS. INTER-SPECIES AND INTER-GENUS EXCHANGE FOR BOTH MODES OF HGT AND INVESTIGATE THE ROLES OF HOSTIMMUNE SYSTEMS (E.G. RESTRICTION/MODIFICATION AND CRISPR-CAS) MISMATCH REPAIR AND RECOMBINATION MACHINERY WHICH ARE ALL KNOWN TO AFFECT HGT IN BACTERIAL MODEL SYSTEMS IN THEIR ABILITY TO INCREASE OR DECREASE THOSE RATES IN AN ARCHAEON.