3 INSERTION SEQUENCES FROM THE CYANOBACTERIUM SYNECHOCYSTIS PCC6803 SUPPORT THE OCCURRENCE OF HORIZONTAL DNA TRANSFER AMONG BACTERIA

Three insertion sequences were characterized from the widely-used cyan obacterium Synechocystis PCC6803. They all harbored a putative transpo sase sequence flanked by two imperfect inverted repeats, seemed to hav e duplicated their target insertion site and occurred as multiple copi es in the host genome. They exhibited no obvious homology with any oth er cyanobacterial ISs and were termed IS5S (871 bp), IS4S (1299 bp) an d ISS1987 (949 bp) because they were, respectively, homologous to IS5- and IS4-bacterial elements, and to several members of the IS63O-Tcl-m ariner superfamily of IS elements occurring in a wide range of hosts. This suggests that these IS-elements were spread through horizontal tr ansfer between evolutionary distant organisms. Three IS5S-copies were isolated as a rescue insertion into a replicating plasmid (IS5Sa), or subsequently cloned from a Synechocystis DNA-library probed with IS5Sa (IS5Sb and IS5Sc), and appeared to be almost identical. In the vicini ty of IS5Sb, we found the ISS1987 element inserted into the ISIS eleme nt. This indicates that the ISS1987 element has been, and could still be, mobile since its transposase sequence is not interrupted with stop codons or translational frameshifts, unlike that which is found in mo st members of the IS63O-Tcl-mariner superfamily of transposable elemen ts. (C) 1997 Elsevier Science B.V.