The Insertion Sequence Element ISRm2011-2 Belongs to the IS630-Tc1 Family of Transposable Elements and is Abundant in Rhizobium Meliloti

The insertion sequence (IS) element ISRm2011-2 of Rhizobium meliloti (Rm) is characterized by 19-bp imperfect terminal inverted repeats (three mismatches) and a size of 1053 bp. Upon transposition, ISRm2011-2 generates a putative target duplication of 2 bp. ISRm2011-2 carries two major overlapping open reading frames (ORFA and B) with a coding capacity of 135 and 201 amino acids (aa), respectively. A potential translational frameshifting window (5'-AAAAAAAG) is located in the overlapping region of both ORFs. The putative fusion product of both proteins, which probably represents the mature transposase, has a predicted molecular mass of 35.8 kDa and a pI of 10.5. Comparison of the deduced aa sequence of ORFA with database entries revealed homology to putative transposases of some IS elements of the IS3 family, as well as to eukaryotic transcription factors. The protein encoded by ORFB shows homology to transposases (Tps) of the recently proposed IS630-Tc1 family which includes Tps of both prokaryotic and eukaryotic transposable elements. Analyses of the distribution of ISRm2011-2 in natural Rm populations showed that this IS element is abundant in Rm strains.