IS1139 from Streptococcus salivarius: identification and characterization of an insertion sequence-like element related to mobile DNA elements from gram-negative bacteria.

An insertion sequence-like element, IS1139, was cloned and sequenced from Streptococcus salivarius ATCC 25975 chromosome. This insertion sequence-like element is 1168 bp long and is delimited by inverted repeats of 29 bp and by a duplicated sequence of 6 bp. This IS possesses an open reading frame that codes for a putative transposase of 339 amino acids which has, respectively, 94, 35, 33, and 30% amino-acid identity with the transposases of IS1161 from S. salivarius ATCC 25975, IS4351 from Bacteroides fragilis, IS30 from Escherichia coli, and IS1086 from Alcaligenes eutrophus. Sequence analysis revealed that these transposases may have evolved from a common ancestral gene. Southern hybridization of restriction endonuclease-digested genomic DNA from 21 strains of oral streptococci, using a probe specific to the transposase-encoding gene (tnpA), revealed that IS1139 is found in two strains of S. salivarius, ATCC 25975 and ATCC 13419, in eight and two copies, respectively.