Abstract
The Sinorhizobium meliloti insertion sequence (IS) elements IS Rm102F34-1 and IS Rm220-13-5 are 1481 and 1550 base pairs (bp) in size, respectively. IS Rm102F34-1 is bordered by 15 bp imperfect terminal inverted repeat sequences (two mismatches), whereas the terminal inverted repeat of IS Rm220-13-5 has a length of 16 bp (two mismatches). Both insertion sequence elements generate a 6-bp target duplication upon transposition. The putative transposase enzymes of IS Rm102F34-1 and IS Rm220-13-5 consist of 449 or 448 amino acid residues with predicted molecular weights of 50.7 or 51.3 kDa and theoretical isoelectric points of 10.8 or 11.1, respectively. IS Rm102F34-1 is identical in 98.9% of its nucleotide sequence to an apparently inactive copy of an insertion sequence element, designated IS Rm7, which flanks the left-end of the nodule formation efficiency ( nfe) region of plasmid pRmeGR4b of S. meliloti strain GR4. IS Rm102F34-1 and IS Rm220-13-5 are closely related since they show an overall identity of 57.0% at the nucleotide sequence level and of 47.3% at the deduced amino acid level of their putative transposases. Both insertion sequence elements displayed significant similarity to the Xanthomonas campestris IS Xc6 and its homolog IS 1478a. Since none of these insertion sequence elements could be allocated to existing families of insertion sequence elements, a new family is proposed. Analysis of the distribution of IS Rm102F34-1/IS Rm7 in various local S. meliloti populations sampled from Medicago sativa, Medicago sphaerocarpa and Melilotus alba host plants at different locations in Spain revealed its presence in 35% of the isolates with a copy number ranging from 1 to 5. Furthermore, IS Rm102F34-1/IS Rm7 homologs were identified in other rhizobial species.
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