The Sinorhizobium meliloti insertion sequence (IS) elements ISRm102F34-1/ISRm7 and ISRm220-13-5 belong to a new family of insertion sequence elements

The Sinorhizobium meliloti insertion sequence (IS) elements ISRm102F34-1 an d ISRm220-13-5 are 1481 and 1550 base pairs (bp) in size, respectively. ISR m102F34-1 is bordered by 15 bp imperfect terminal inverted repeat sequences (two mismatches), whereas the terminal inverted repeat of ISRm220-13-5 has a length of 16 bp (two mismatches). Both insertion sequence elements gener ate a 6-bp target duplication upon transposition. The putative transposase enzymes of ISRm102F34-1 and ISRm220-13-5 consist of 449 or 448 amino acid r esidues with predicted molecular weights of 50.7 or 51.3 kDa and theoretica l isoelectric points of 10.8 or 11.1, respectively. ISRm102F34-1 is identic al in 98.9% of its nucleotide sequence to an apparently inactive copy of an insertion sequence element, designated ISRm7, which flanks the left-end of the nodule formation efficiency (nfe) region of plasmid pRmeGR4b of S. mel iloti strain GR4. ISRm102F34-1 and ISRm220-13-5 are closely related since t hey 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. Bo th insertion sequence elements displayed significant similarity to the Xant homonas campestris ISXc6 and its homolog IS1478a. Since none of these inser tion sequence elements could be allocated to existing families of insertion sequence elements, a new family is proposed. Analysis of the distribution of ISRm102F32-1/ISRm7 in various local S. meliloti populations sampled from Medicago sativa, Medicago sphaerocarpa and Melilotus alba host plants at d ifferent locations in Spain revealed its presence in 35% of the isolates wi th a copy number ranging from 1 to 5. Furthermore, ISXm102F34-1/ISRm7 homol ogs were identified in other rhizobial species. (C) 1999 Federation of Euro pean Microbiological Societies. Published by Elsevier Science B.V. All righ ts reserved.