A Coxiella burnetti repeated DNA element resembling a bacterial insertion sequence

A DNA fragment located on the 3' side of the Coxiella burnetii htpAB operon was determined by Southern blotting to exist in approximately 19 copies in the Nine Mile I genome. The DNA sequences of this htpAB-associated repetitive element and two other independent copies were analyzed to determine the size and nature of the element. The three copies of the element were 1,450, 1,452, and 1,458 bp long, with less than 2% divergence among the three sequences. Several features characteristic of bacterial insertion sequences were discovered. These included a single significant open reading frame that would encode a 367-amino-acid polypeptide which was predicted to be highly basic, to have a DNA-binding helix-turn-helix motif, to have a leucine zipper motif, and to have homology to polypeptides found in several other bacterial insertion sequences. Identical 7-bp inverted repeats were found at the ends of all three copies of the element. However, duplications generated by many bacterial mobile elements in the recipient DNA during insertion events did not flank the inverted repeats of any of the three C. burnetii elements examined. A second pair of inverted repeats that flanked the open reading frame was also found in all three copies of the element. Most of the divergence among the three copies of the element occurred in the region between the two inverted repeat sequences in the 3' end of the element. Despite the sequence changes, all three copies of the element have retained significant dyad symmetry in this region.