ISD1, AN INSERTION ELEMENT FROM THE SULFATE-REDUCING BACTERIUM DESULFOVIBRIO-VULGARIS HILDENBOROUGH - STRUCTURE, TRANSPOSITION, AND DISTRIBUTION

Insertion element ISD1, discovered when its transposition caused the i nsertional inactivation of an introduced sacB gene, is present in two copies in the genome of Desulfovibrio vulgaris Hildenborough. Southern blot analysis indicated at least two insertion sites in the sacB gene , Cloning and sequencing of a transposed copy of ISD1 indicated a leng th of 1,200 bp with a pair of 44-bp imperfect inverted repeats at the ends, flanked by a direct repeat of the 4-bp target sequence, AAGG and AATT were found to function as target sequences, ISD1 encodes a trans posase from two overlapping open reading frames by programmed translat ional frameshifting at an A(6)G shifty codon motif, Sequence compariso n showed that ISD1 belongs to the IS3 family, Isolation and analysis o f the chromosomal copies, ISD1-A and ISD1-B, by PCR and sequencing ind icated that these are not banked by direct repeats, ISD1-A is inserted in a region of the chromosome containing the gapdh-pgk genes (encodin g glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate kinase ), Active transposition to other loci in the genome was demonstrated, offering the potential of a new tool for gene cloning and mutagenesis, ISD1 is the first transposable element described for the sulfate redu cers, a large and environmentally important group of bacteria, The dis tribution of ISD1 in genomes of sulfate-reducing bacteria is limited, A single copy is present in the genome of D. desulfuricans Norway.