Analysis of spontaneous mutations in revealed a 1237 bp insertion sequence named IS and possessing a nucleotide sequence resembling those of members of the IS3 family. The chromosome of strain ADP1 contains seven copies of IS which appears to insert preferentially into the transcriptional activator of the structural gene for -hydroxybenzoate hydroxylase. IS creates tandem 3 bp DNA duplications flanking the sites of its insertion in Different duplication patterns are found following insertion of IS into a structural gene for protocatechuate 3,4-dioxygenase. Therefore the insertion properties of IS appear to be influenced by its DNA target. Amino acid sequences associated with the apparent transposase function have been conserved in ORFB of IS whereas the presumed DNA-binding helix-turn-helix region of IS ORFA exhibits substantial amino acid sequence divergence from its IS3 counterparts. IS ORFA and ORFB coding sequences overlap considerably, and sequence evidence indicates mechanisms for ORFB expression in IS may resemble those employed by other members of the IS3 family. Portions of the IS terminal repeats exhibit substantial sequence divergence from other members of the IS3 family, but evolution appears to have conserved a mechanism preventing expression of the insertion sequence genes as a consequence of transcriptional readthrough.


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