LONG STRETCHES OF SHORT TANDEM REPEATS ARE PRESENT IN THE LARGEST REPLICONS OF THE ARCHAEA HALOFERAX-MEDITERRANEI AND HALOFERAX-VOLCANII AND COULD BE INVOLVED IN REPLICON PARTITIONING

We report the presence of long stretches of tandem repeats in the geno me of the halophilic Archaea Haloferax mediterranei and Haloferax volc anii. A 30 bp sequence with dyad symmetry (including 5 bp inverted rep eats) was repeated in tandem, interspersed with 33-39 bp unique sequen ces. This structure extends for long stretches - 1.4 kb at one locatio n in H. mediterranei chromosome and about 3 kb in the H. volcanii chro mosome. The tandem repeats (designated TREPs) show a similar distribut ion in both organisms, appearing once or twice in the H. volcanii and H. mediterranei chromosomes, and once in the largest, probably essenti al megaplasmid of each organism but not in the smaller replicons. Sequ encing of the structures in both H. volcanii replicons revealed an ext remely high sequence conservation in both replicons within the species , as well as in the different organisms. Homologous sequences have als o been found in other more distantly related halophilic members of the Archaea. Transformation of H. volcanii with a recombinant plasmid con taining a 1.1 kb fragment of the TREPs produced significant alteration s in the host cells, particularly in terms of cell viability. The intr oduction of extra copies of TREPs within the vector significantly alte rs the distribution of the genome among the daughter cells, as observe d by DAPI staining. Although the precise biological role cannot be com pletely ascertained, all the data conform with the tandem repeats bein g involved in replicon partitioning in halobacteria.