Genetic organization of the mobilization region of the plasmid pHE1 from Halomonas elongata

The mobilization (mob) region of the non-self transmissible 4.2-kb plasmid pHE1 from the moderately halophilic bacterium Halomonas elongata ATCC 33174 has been identified and characterized. Analysis of the sequence revealed t he presence of four open reading frames (mobCABD) which show a complex orga nization with two of them (mobB and mobD) entirely overlapped by a third (m obA). The deduced proteins appeared to have a high degree of homology to Mo b proteins of CoIE1 and closely related plasmids. To assess the functionali ty of the mob region, the hybrid vector pHS134 was constructed, consisting of the complete plasmid pHE1, the E. coil vector pKS(-) and a streptomycin- resistance gene for positive selection in Halomonas. Vector pHS134 was foun d to bs mobilizable from E. soli to It elongata assisted by pRK600. Upstrea m of the mob genes, an oriT region with a putative nick sequence highly hom ologous to that of CoIE1 plasmids was identified. To our knowledge, this is the first mobilizable plasmid found in moderate halophiles. This property, together with its small size, the availability of its complete sequence, a nd its broad host range: in moderately halophilic strains, makes pHF1 a goo d candidate for the construction of cloning and expression vectors for thes e extremophiles.