Identification and nucleotide sequence of Rhizobium meliloti insertion sequence ISRm6, a small transposable element that belongs to the IS3 family.

The insertion sequence ISRm6 is a small transposable element identified in Rhizobium meliloti strain GR4 by sequence analysis. Two copies of this IS element were found in strain GR4, one of them is linked to the nfe genes located on plasmid pRmeGR4b. ISRm6 seems to be widespread in R. meliloti. Data suggest that ISRm6 is active in transposition at an estimated frequency of 2 x 10(-5) per generation per cell in strain GR4. This 1269-bp element carries 27/26-bp terminal imperfect inverted repeats with six mismatches and a direct target site duplication of 4 bp. The IR terminate with the dinucleotide 5'-TG as all the members of the IS3 family. In addition, as other IS belonging to the IS3 family, ISRm6 carries two open reading frames (ORFA and ORFB) with a characteristic translational frame-shifting window in the overlapping region. Furthermore, ISRm6 putative transposase contains the triad of amino acids called DDE motif. Comparison of the ISRm6 DNA sequence and the putative proteins encoded with sequences derived from the EMBL, GenBank, PIR and Swissprot databases showed significant similarity to IS that belongs to the IS3 family with a highest homology to a subclass containing IS476 from Xanthomonas campestris, IS407 from Burkholderia cepacia, and ISR1 from Rhizobium lupini.